If it functions OK and is still covered by a repair warranty then don't worry about it.
BTW - the "internal serial number" in the EXIF data is not the camera's actual serial number as depicted on the external badge. 😉
Plastic mounts are fine and are sufficiently strong ......... unless you are ham-fisted and/or mistreat your equipment. Never had an issue with any of mine. 😉
Your X-T10 and X-T2 are probably well worn by now and lenses will probably feel more loose on them than on a newer body. Also, having a plastic mount, the XC 35 generally feels not as tight as metal mount lenses. Don't worry about it. 😉
No longer have an X-T3, but my X-H1 appears similar if not a bit better compared to my X-T20. Check your screen settings for brightness / color / adjustments.