Sorry for the late reply.
I don’t think hiding or showing the list is what causes the problem.
The only reliable way I’ve found to diagnose it is by repeatedly tapping one or all of the buttons. If I tap a button at about 4 times per second (like 8th notes in 4/4 at 120 BPM), I can track the pattern pretty easily. It varies between roughly 12–16 taps before the issue appears. That means it works for about 3–4 seconds, then stops responding for about 3–4 seconds, then works again, then stops again… and this cycle continues as long as I keep tapping the button.
The same pattern appears when I alternate between the three buttons on each click, and I can’t detect any difference in their behaviour in this regard.
I can say for sure it always start "not working" for about 3-4 sec when I start or switch to ST4.
If this issue doesn’t occur on your iPhone, Peter, then it’s clear that not all iPhones are affected, which indicates that the cause may lie outside of both iOS and ST4.
But this is where it gets really tricky to pin down.