Battery issues after housing replacements

We’ve repaired housings on iPhone’s for quite a while now, all of a sudden recently we are finding that we are having battery issues after repairs, it seems isolated to the iPhone 8. Even if we change the battery with other used genuine Apple batteries we still have the same issues, I’ll list the problems below;

1) Phone charges really quickly. 2) Battery life drains rapidly. 3) When we attempt to drain the battery they die at around 3% rather than going down to 1% and turning off. 4) When we attempt to charge them back up from dead, they immediately turn back on rather than taking the usual amount of time to turn on and are sat at 3%. We try different batteries every time and its the same issues. I’m thinking board problem but its happening quite often and I’m wondering if its the techs are doing when changing the housing or if we are just getting unlucky batches.