Hmm, annoying. It seems I cannot reproduce it or I do something different from you.
What I did is add 5 services to the Form 1 that is there after install of the app booking plugin. They use different time durations.
Then on the page with the shortcode, I also add the [gwolle_gb] shortcode.
I can still select a service and it gives me the correct timeslots. When selecting a time it will show the desired slot.