BBC region studios opt into and out of the network sustaining feed seamlessly. Thus you get the local presenter able to be inserted into the 6 O'Clock News with a 'local headline, coming up later' type bit...
They should opt back at the end credits/trails and usually do so.
Once upon a time the circuits all went through the regional studios. Now, I understand, the switching is done remotely in the Code & Mux centres, but under local studio control, and sound/pictures will be mixed in their studio control rooms at times.
It's possible that any opt into or back to network might disrupt a transport stream? I was 'thinking out loud'. But other evidence now points to the Network Playout being at fault, so now a very red herring.
(In olden analogue days one could sometimes observe a timing issue where the picture shifted when the region opted the studio in/out).