I added 3 fingerprints to unlock my FP5 since months.
Some weeks ago, one of my fingers cannot unlock the FP5. After investigation, I saw that only 2 fingerprints remained, one has disappeared. I re-registered the one missing. So, last week, I had 3 fingerprints registered.
This week, the issue is back; I lost once again my 3rd fingerprint. I do not know if there is a link, but I had a /e/os update to 3.1.1.
Since you are running /e/OS, you may find more information in the Murena forum.
While I haven’t seen actual deletions of registered prints myself, it’s possible that you are seeing a “new” feature of Android 15.
However, that would not be the case if you saw this before updating to A15, of course.
If you have registered several different fingers, it’s probably good practice to use all of them regularly, and not just use one on a daily basis.