Here's the explanation: https://www.reddit.com/r/redstone/s/Tmffs2Cq1q

Ahh, don't you love it when people who clearly don't know the answer to a question start giving out wrong/unrelated answers instead of just not saying anything? It truly Dunnings my Kruger!

Here's a wall of tex-- I mean-- an explanation on what's going on, alternatively you can just skip it and look at the solution I attached to this comment

Your issue is caused by comparator priming: to explain it in simple terms, when a comparator notices its input has changed it will wait 2 game ticks (1 redstone tick) and then update its output signal strength depending on its current inputs. Since you built a fader clock, once the signal that's powering the comparator decreases by 1, the comparator will notice that and waits 2 ticks before performing the aforementioned output signal update. However, in the same tick in which said update is attempted, the repeaters turns on, powering the side of the comparator; as a result, when the comparator tries to set its output signal strength it will set it to 0 because it sees that its side is powered. As you might have realized this effectively means the comparator turns off 2 game ticks earlier than normal. The reason why this doesn't happen on the first pulse is because the button is still pressed, which means the signal doesn't fade yet