6S No charging symbol, still charging up but not past 11%

Hi there, this is my first time posting on iFixit as I've run into an issue I haven't seen before.

I have a customers iPhone 6S which isn't charging, I've tested charge port and battery to no avail, and also tried device in a known good housing assembly.

The device charges with no charging symbol or sound, however it only charges up to 11% (roughly) and stops. I left it on charge overnight and arrived one morning to find it on 6ish%, hence my confusion.

Would this be tristar? How would I know, and what can I test to find out?

Device has not been water damaged, and came in for a screen replacement initally.

Have done a mainboard inspection and can't see any obvious burns or damage.

Kind regards,

Silas.