Apple Users

If I were you I'd power off (not just lock) the phone and turn it back on after a bit. Sounds a little like the chip that supervises the battery might need a swift kick.

Btw in newish iOS versions, the "Settings" app has a whole entry devoted to "Battery". It might be useful to keep an eye on what it is reporting there.

Of course you could also try using a different lightning cable from the power adapter to the phone & checking the lightning receptacle for dirt.

But .... "techs who installed the battery" ... if not done by Apple, I hope they were at least Apple Certified