Seemingly random behavior for Find Usages

My custom language plugin implements References and a FindUsagesProvider for numeric and string variables, where the latter all end in a '$' character.                                              
                                                                                                    
For unknown reasons, I experience some very weird behavior when finding usages for such string variables: Seemingly at random, finding usages works for some string variables, but not for others:

For all variables, however, IDEA highlights all variable instances when placing the cursor on any instance.

I do not understand how find usages can work for some variables, but not for others.  I've ruled out that the position in the Psi tree matters: For example, renaming (manually, without using inline renaming) a working variable can result in a non-working variable.  

Also, I've never found an instance where find usages didn't work for numeric variables, so I think the '$' character does play a role.

This is my FindUsagesProvider

public class Xbas99FindUsagesProvider implements FindUsagesProvider {

    @Override
    @Nullable
    public WordsScanner getWordsScanner() {
        return new DefaultWordsScanner(new Xbas99LexerAdapter(),
                TokenSet.create(Xbas99Types.IDENT, Xbas99Types.SIDENT),  // identifiers
                TokenSet.create(Xbas99Types.COMMENT),  // comments
                TokenSet.create(Xbas99Types.QSTRING, Xbas99Types.NUMBER, Xbas99Types.FLOAT));  // literals
    }

    @Override
    public boolean canFindUsagesFor(@NotNull PsiElement psiElement) {
        return psiElement instanceof Xbas99NamedElement;
    }

    @Override
    @Nullable
    public String getHelpId(@NotNull PsiElement psiElement) {
        return null;
    }

    @Override
    @NotNull
    public String getType(@NotNull PsiElement element) {
        if (element instanceof Xbas99Linedef || element instanceof Xbas99Lino) {
            return "BASIC line number";
        }
        if (element instanceof Xbas99NvarW || element instanceof Xbas99NvarR) {
            return "BASIC numerical variable";
        }
        if (element instanceof Xbas99SvarW || element instanceof Xbas99SvarR) {
            return "BASIC string variable";
        }
        return "";
    }

    @Override
    @NotNull
    public String getDescriptiveName(@NotNull PsiElement element) {
        return getNodeText(element, true);
    }

    @Override
    @NotNull
    public String getNodeText(@NotNull PsiElement element, boolean useFullName) {
        if (element instanceof Xbas99Linedef) {
            return ((Xbas99Linedef) element).getName();
        }
        if (element instanceof Xbas99Lino) {
            return ((Xbas99Lino) element).getName();
        }
        if (element instanceof Xbas99NvarW) {
            return ((Xbas99NvarW) element).getName();
        }
        if (element instanceof Xbas99NvarR) {
            return ((Xbas99NvarR) element).getName();
        }
        if (element instanceof Xbas99SvarW) {
            return ((Xbas99SvarW) element).getName();
        }
        if (element instanceof Xbas99SvarR) {
            return ((Xbas99SvarR) element).getName();
        }
        return "";
    }

}

and my Reference.

public class Xbas99VarReference extends PsiReferenceBase<PsiElement> implements PsiPolyVariantReference {
    private final String dummy = CompletionUtilCore.DUMMY_IDENTIFIER_TRIMMED.toUpperCase();
    private final String varName;
    private final boolean isStringVar;

    public Xbas99VarReference(@NotNull PsiElement element, TextRange textRange, boolean forString) {
        super(element, textRange);
        varName = element.getText().substring(textRange.getStartOffset(), textRange.getEndOffset()).toUpperCase();
        isStringVar = forString;
    }

    @Override
    public ResolveResult @NotNull [] multiResolve(boolean incompleteCode) {
        List<ResolveResult> results = new ArrayList<>();
        final List<Xbas99NamedElement> vars = isStringVar ?
                Xbas99Util.findSvardef(myElement, varName, false) :
                Xbas99Util.findNvardef(myElement, varName, false);
        for (Xbas99NamedElement var : vars) {
            results.add(new PsiElementResolveResult(var));
        }
        return results.toArray(new ResolveResult[results.size()]);
    }

    @Override
    @Nullable
    public PsiElement resolve() {
        ResolveResult[] resolveResults = multiResolve(false);
        return resolveResults.length == 1 ? resolveResults[0].getElement() : null;
    }

    @Override
    public Object @NotNull [] getVariants() {
        boolean queryForString = false;
        String varText = varName;
        if (varText.endsWith("$")) {
            do {
                varText = varText.substring(0, varText.length() - 1);  // remove last char
            } while (varText.endsWith(" "));
            queryForString = true;
        }
        if (varText.endsWith(dummy)) {
            varText = varText.substring(0, varText.length() - dummy.length());
        }
        List<Xbas99NamedElement> vars = isStringVar || queryForString ?
                Xbas99Util.findSvardef(myElement, varText, true) :
                Xbas99Util.findNvardef(myElement, varText, true);
        List<LookupElement> variants = new ArrayList<LookupElement>();
        for (final Xbas99NamedElement var : vars) {
            if (!var.getName().isEmpty() && var.getName().length() > 0) {
                variants.add(LookupElementBuilder.create(var).
                                withIcon(Xbas99Icons.FILE).
                                withTypeText(var.getContainingFile().getName())
                );
            }
        }
        return variants.toArray();
    }

    @Override
    public PsiElement handleElementRename(@NotNull String newElementName) throws IncorrectOperationException {
        Xbas99PsiImplUtil.rename(myElement, newElementName);
        return myElement;
    }

}

The entire project together with some examples are on GitHub: https://github.com/endlos99/plugin-wip.git
The code in question is in directories xbas99 and xbas99l (two variants of the same language, both are affected).