Hi @Appajipjip
Welcome to the Sonos Community! And, apologies for the delay.
I consulted with colleague, and I think we have a picture of what is happening here.
An important point is that Ace cannot play from the two Bluetooth devices at the same time - one feed must stop before another can be used.
When you take off the headphones, the iPhone is pausing, as it should. However, as the MacBook is also connected, it is “stealing” Ace’s attention, which then switches to the MacBook feed - this might be happening due to audio activity on the MacBook. This means that, in order to then play from the iPhone again, you have to stop the feed from the MacBook first.
Do you have another phone or tablet? I suspect that if you were to test with two such devices (rather than use the MacBook) you would not see this “stealing” behaviour anymore - we’d like to ascertain whether the issue is happening as a result of the MacBook constantly outputting audio or not. If you can test this and get back to me, that would be very helpful.
It does seems to me, however, that source switching should be disabled when the Ace is removed from your head - I’ll mark this thread as feature request.
I hope this helps you understand what is happening.