There is an open merge request for messagelib: https://invent.kde.org/pim/messagelib/-/merge_requests/100 which I would love to have fixed.
I am hitting this crash nearly daily. If you cannot reproduce this I can give you my pubring.kbx which is large enough to make this issue easily reproducible even on very fast computers.

@dvratil I am assigning this directly to you even though that is not the usual workflow.