Getting the same and changing fingerprints to 2+2 seems to do the trick on a P8 with the latest updates.
What's odd though is that I had 3+1 and I believe it worked although I'm not 100% sure that I added the 3rd fingerprint of the owner profile before the last update(s). I first tried 3+3, did not work.
2+2 seems to be working.
This is weird, I'm getting the opposite. If I end the session of my secondary user, I can use fingerprint on the owner (even with > 2 fingerprints on the owner), but if I switch between both, I only can if I have 2+2.