Your observation that the state changes cleanly rather than flickering is the detail that matters, and it rules out the loose-port diagnosis most people would reach for. A cracked connector gives you an intermittent, flickering charge state. A clean stop at a repeatable level is a decision being made by the charge circuit.
That circuit monitors the cell's voltage and temperature and stops when either goes outside its safe window. As a lithium cell ages, its internal resistance rises. Higher resistance means more heat for the same charge current, and it also means the voltage at the cell terminals climbs faster than the actual state of charge, so the circuit sees what looks like a nearly full battery, or an uncomfortably warm one, and stops early. Stopping around 80% is a very typical presentation of this at two years of daily charging. It is the protection working correctly on a cell that is no longer what it was, rather than the protection malfunctioning.
Two quick things to rule out before concluding. Charge it somewhere cool with the controller switched off, because charging a warm controller while playing is a common reason for an early stop, and that one is free to test. And try charging from a plain low-power USB port rather than a fast charger, since a lower current generates less heat and can get past a marginal cell.
If it still stops at 80% cool and off, the cell is the answer. A DualSense battery is an inexpensive part on a plug-in connector with no soldering, and it is one of the easiest repairs on this controller. Replacing it will restore full charging and also fix the shortened runtime you are probably getting without having noticed, since the two go together.
And the standard safety point: if the battery shows any swelling or has lifted the shell, stop charging it and dispose of it at a recycling point rather than working on it.
