Trigger on Firestore-document-update not triggering with document-filter

I am trying for a few hours now and I can't figure it out - maybe somebody can give me a hint.

I am trying to set up a trigger, that a Cloud Function is triggered, when a document in my Firebase "Answers"-collection is updated. I set up Eventarc google.cloud.firestore.document.v1.updated; database=(default) - and it works, but only when I don't use a document-filter!

As soon as I type a filter in (it offers "document" in the GUI), nothing is triggered. As filter I basically use what's in the logs, when the function is actually running, so I don't think it can be wrong.

I already tried:

Answers/*
Answers/{answer}
documents/Answers/{answer}
projects/myProjectId/databases/(default)/documents/Answers/{answer}
...

(myProjectId is of course my project id)

I can't figure it out...anybody has an idea?
Thanks a lot!