I was not able to reproduce it in a minimal example, so I copied my Tasks with the issue and started to deactivate more and more Actions till I found the culprit!
In my Task there was a Flash Action in the beginning, quite useless though, saying that my Task starts. With a timeout of 1 sec only and with Continue Task Immediately: yes.
It had also the ID: 1 as all my other Flash Actions.
When I disable this Flash Action or if I delete the ID (or set it to another value than 1) or if I set
Continue Task Immediately: no or if I set the Timeout to more than only 1 sec - than that later Flash Action with ID:1 which was not disappearing, suddenly disappears as expected, when the Flash Action with ID:1 of the other Task runs.
So somehow if there are two Flash Actions with the same ID and both
set to Continue Task Immediately: yes and the first one has a short Timeout (1sec or less) then the second one could be not reached by ID.
But I cannot reproduce it with a simple Task, so I think it happens also because my Task is too long (contains some Wait in Loop Actions) and somehow the ID of the second Flash Action is mixed up with the first one.
This might be a bug, I guess. Maybe it is simpler than that, but I didnt find it and give up now.
Now that I know how it happens, it is no issue to me anymore, thanks :)
frank