No you’re breaking the power bank, power left in a battery is determined by the voltage as in the way it’s measured is by checking it’s voltage output against some math when knowing the top and bottom of its charge state outputs. If being plugged into itself “charges it” that means the voltage output is increasing without the charge held increasing.
Normally in a decent quality power bank this would do literally nothing, either it wouldn’t let it happen or it would just slowly lose charge normally as it pumps electricity into itself at a lose due to heat. The fact this is happening basically means you’ve effectively forced the battery to age faster than expected, by that I mean cause normal wear and tear expected over thousands of cycles in a very short time, and as such hold less total charge.
Normally in a decent quality power bank this would do literally nothing, either it wouldn’t let it happen or it would just slowly lose charge normally as it pumps electricity into itself at a lose due to heat. The fact this is happening basically means you’ve effectively forced the battery to age faster than expected, by that I mean cause normal wear and tear expected over thousands of cycles in a very short time, and as such hold less total charge.