I thought I was going crazy, but this must be a bug.
In the after effects beta - with a custom layout - I am losing my timline panel - and there is not an option to reopen it.
Every time i choose my custom layout i lose the timeline and there is no menu option to re open it.
This seems to have started when i accidently saved over the default layout when trying to restore the default layout. I had to delete the beta and re install to get the default settings to reset. I think my settings were also syncing to creative cloud so there was a feedback loop keeping me from fully resetting them. I've lost track of the details - but ive fully lost the default layout, and when i choose the all windows layout - i have this same issue losing the timeline panel and no way to restore it.