x-pve Your tablet is brand new, therefore a battery defect is more unlikely... (Not impossible) However, the dock is suposed to charge slowly (in my experience) while a 25-30W adapter charges alot faster, perhaps it "crawls" slowly to a 100% while docked? (Especially the latter percentages it takes its time to prevent heat)
It does heat up the battery less prominently through the dock however, which is a good thing.
Perhaps you can test out if it reaches 100% by turning it completely off and let it charge on a USB adapter, just give it some time to charge, batteries can be a bit quirky sometimes.
Sadly, this is as far as my knowledge on batteries go, perhaps anyone else has an idea?
And did you turn off charge optimisation under battery settings? (Which limits the charge to 80%, which can be a good thing for your battery on the long run)