I have now tried a lot on two different phones.
On the first (Samsung A55) nothing happens with single or double tap.
On the other (Samsung A23), if I single tap once and then again on the same marker, it does open the update form.
And now I can consistently open a form by single click.
I have now also tested on an android tablet, and here it works as expected.
Tooltips are newer shown on any device.
On the unresponsive phone I cannot open a cluster of markers either.
So it seems, that this phone generally is less responsive to clicks. I have tried fiddling with the display settings, but nothing helps.
But it seems to be phone specific.
Have no Apple devices to test with.