To put is short: I had to reboot the phone, but it seems working fine after it.

Installed it, started the app and the music in foreground, headset button started my last used (factory) music layer. Then I turned media control on in xiia, but it did not change anything. Restarted the phone, started tunein radio to see if headset is ok with it, it did work well. Started xiia and headset buttons worked well in xiia this time, and it is working well since then. However I do not see any effect of turning media control on or off in xiia, the last used music player starts in both case, if I press the headset button when no music app is running.