[Pete]

Your explanation might make sense, but I don't know.

I now set up a MIDI button. Action:  send MIDI command - patch select. The effect is:  if I press the button the first time the GT shows the correct preset, then the first preset, then the correct preset again and so on. very strange.

I use the same Midi-interface cable. But there is a different adapter cable between, because the USB-connection at the tablet is different. The new tablet has USB-C, the old on has USB-B micro, so I have to use a different adapter.