I ran into this problem and I was able to fix the problem by compiling kbibtex from upstream. I believe that the problem was fixed by this merge: https://invent.kde.org/office/kbibtex/-/merge_requests/11 which indeed works around a bug/changed behavior of recent versions of Qt.
Unfortunately, it seems that this fix did not make it into debian because the project has not made a release since then I believe (I am a bit confused by the tags in the repository). I believe that this could be fixed either by taking the last version of the 0.9 branch of upstream or asking upstream to do a release. Amaury
