Code Completion Results not showing

Hello,

I am writing a custom code completion contributor, but one of my providers just won't work.
I have multiple different CompletionProvider which work fine, but the results of one CompletionProvider won't show up.

When debugging, I see that the provider itself finishes and adds a list of >50 results to the CompletionResultSet.
I tried commenting out my other custom providers and to debug the CompletionService.

Debugging the CompletionService resulted in unexpected behavior.
CompletionService.getVariantsFromContributors iterates over all contributors.
However, it seems, that at some point the progress is being canceled.
But it is not always the same one being canceled, and it is not my custom contributor which is canceled.

Do you have any hints for me to solve this problem? What are possible reasons that default IDE completion contributors are being canceled? Why are my other custom completion providers working, but one leads to problems?
Why are my results added to the CompletionResultSet, but not shown in the editor?