I don't recall the details, but I think it was when I first switched from 17.1 lineage with TWRP, to 18.1 with Lineage recovery, that the phone would restart to the PIN entry screen but would not accept my PIN. In the end after trying various workarounds found online, I had to wipe the phone and start from zero. I seem to recall others having a similar issue, maybe discussed earlier in this thread. So definitely back up as much as you can preferably manually to be sure (seedvault unfortunately excludes a lot for me at least), and I think somewhere I saw someone suggest removing the PIN and fingerprint lock before you do the upgrade, but then someone earlier in this chain seems to say they had the same issue even having done that.