Inside a cheap basic lithium charger from China

It won't, because once fully charged, the cell cannot accept any more charge. But if the charger has not fully terminated it may still be applying a voltage to the cell. If so, it can cause plating of metallic lithium on the anode. This is invisible, can not be detected by a DMM and is dangerous because as it accumulates over time it causes the cell to become unstable.

This is where the problem is. Doing it once or twice likely won't be a problem. But if it happens repeatedly the cell is being damaged as described above.

I beat this subject to death a couple years back, so I won't do it again. You can even do a simple practical test to see if your charger is continuing to apply a voltage to the cell after termination. See this post and some of the discussion prior to it for more info.