Comparator in subtraction mode does not update visually under certain conditions

Situation:
A redstone comparator in subtraction mode, fed with the same signal strength to the input (back) and one of the sides (left/right), stays visually on if the input signal turns off before the side signal does.

Steps to reproduce:
1. place a comparator
2. put it in subtraction mode
3. feed it any signal strength to the input
4. feed it the same signal strength to the left/right
5. output turns off, comparator torches stay on
(comparator "visually on" though it's not outputting any redstone signal)
6. turn off the signal to the input, output stays off, torches stay on
7. (optional) turn off the signal to the side, torches still stay on
8. (optional) remove everything except the comparator. torches still stay on

Expected behaviour:
I expected the comparator torches to turn off at (5).

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Code analysis by Marcono1234 here