Hey SputnikCZ, so your ResBT is fine for me - doesn't disappear at all. Your ProducBT button does disappear though .
When I'm debugging, I usually do two things - I first use the search in the events tab at the top of construct to search for the object which is causing the problem. This helps catch any stray events which may be the issue.
If that doesn't help I fire up the game in debugging mode and watch exactly what happens to the object as the problem occurs.
In this instance I searched and couldn't find anything, so I watched the object in the debugger. I saw that it had been destroyed by the fade behaviour . The debugger is really useful!
Hope that helped