Fingerprints not recognized after screen replacement

So I'm on my second (yes, I know) screen replacement...the first time I got it back with the dreaded message "Can't use fingerprint sensor. Visit a repair provider or g.co/pixel/fingerprintunlock" but thanks to the instructions on ifixit it was a breeze to fix. The shop had left the new sensor in place.

The second time is...today and this time the shop (another one) reused the old sensor, saying that the new one caused some "flickering" (not clear exactly what it was, could be the "flickering" I saw before I recalibrated the sensor the first time). So no need to recalibrate with the tool (I did it all the same just in case) but my fingers were not recognized. I deleted all the old ones just in case and added some new ones (with difficulty I might add, frequent time outs...) but no go.

So what's strange is that the sensor seems to work (it can memorize new fingers) but then it fails to recognize them to unlock the phone (or log into fingerprint protected apps). I saw a tip about booting into safe more, deleting and re-adding, no go either. Now there was water damage (yes the P6 is no longer waterproof after the screen's been changed.) so it's possible/likely the fingerprint sensor got damaged too.

So has anyone seen that problem before ? My only option at this point is to go back to the shop later this week and have them put back the new sensor (and hoping it didn't get damaged in the process). If anyone's had the same problem before I'd be happy to try any tips, thanks !