6S Plus Rear Camera - Left side not focussed

Indeed, this could be a bad replacement camera…or a logic board issue. The challenge when troubleshooting iPhone issues is that you need to have access to high quality, known good parts. Unfortunately, Apple doesn’t want to play ball here and refuses to distribute high quality replacement parts to third party shops. Some vendors, like iFixit, do a tremendous job at quality control but the whole “iPhone replacement part” ecosystem suffers from crap parts noodling their way through the supply chain.

Beyond trying another camera, my feeling here is that one of the many voltages used to power the camera and the OIS system may be damaged. A little probing would confirm this possibility so if you have a local shop that does micro-soldering work, they may be able to help out.