Custom EL variable marked as unresolved in Ultimate despite PsiReferenceContributor

We have some custom objects that we keep in session for users. Typically we add the @elvariable comment annotation, but some of the pages get quite messy.

We're experimenting with a PsiReferenceContributor to resolve these - the reference contributor works in that you can click through and you get IntelliSense on it, but the variable itself is still highlighted in yellow with the "Cannot resolve variable" warning.

Is there another contributor we need to create to allow this warning to disappear?