Create a new presentation.
Add a couple of objects to the slide 1 so it has different contents than the master slide.
Change to master slide.
Add new object to master slide.
Leave master slide.
Undo the "Add Instance" -> Slide changes to master as expected, but now you can no longer leave the master slide using "leave master" button. The slide panel shows you have left the master if you click on "leave master", but the timeline contents do not change and the timeline doesn't get notification about slide change, so effectively you are still on master slide.
To reset the state, you need to manually change to a different slide by clicking on Slide 1.
Note that this is not a new timeline issue, the same problem exists in pre-timeline changes version.