@david_chisnall thanks, I'll try suggesting they try that button the next time I see this happen for someone, but why doesn't it sync automatically?
I usually present a PDF or HTML from my Mac, so I never experienced the PowerPoint split slides problem as a presenter, just as an audience member (most recently it happened last week I think).

Also I'm not sure what the best way to report this bug would be. With an open source project I'd open an issue directly on their bugtracker, but with this one would probably have to open a ticket with internal IT, have them reproduce it, potentially talk to the vendor on my behalf, etc. So I think most of these bugs go unreported (we just assume that MS would be internally using this and is well aware of these bugs, just had other priorities than fixing them, although that may not be the case).
Other bugs (like room audio not working) do get reported and fixed, because those are very easy to reproduce.

@edwintorok

I'll try suggesting they try that button the next time I see this happen for someone, but why doesn't it sync automatically?

Because if I have decided to move to a different slide to go back over a point that you made earlier and having you advance the slide lost my page, that would be staggeringly annoying. Doubly so if you're using morph transitions or similar to create build effects, because now you're advancing slides every ten seconds and you keep losing my place. It shows a big red 'sync to presenter' button to show you that the presenter has moved on, but doesn't force you to move on if you've already interacted with the UI to express an intention to be on a different slide from the presenter.

Also I'm not sure what the best way to report this bug would be. With

No one knows. Even when I worked at Microsoft, I never managed to successfully report a bug in one of their products (which didn't stop them from forcing me to use pre-release alpha versions of things). The only exception were things like the Windows Terminal, which had GitHub issue trackers.