I had the same problem. I use magisk (twrp install) and opengapps and every second OTA update failed. Means after lineageos start (lineageos bootscreen only) it aborts after about 3 minutes and boots into twrp. Reproducible at every second OTA. Then i see magisk doesn't refresh at OTA update. The solution is to install magisk in twrp again. But this is only a side problem because lineageos doesn't start either then. I must install opengapps already in twrp again. Then the phone start fine without problems.
I don't know the reason for that, but it solves the problem every time without wiping anything.