Playhead stops upon adding a default transition

New poster here. I've lurked for a while monitoring all the potential solutions to this problem of the playhead stopping at a certain random point. For some it was disabling the audio process, (mine could not be force-quit) for some it was reverting to the default layout, for some simply double-clicking on the project.

None of those worked for me. 

The only thing I've found that works is to force-quit the program and begin again. But invariably I reflexively add a transition with the keyboard shortcut and the whole thing starts over again.

Obviously this isn't the only thing that triggers the bug. It's just the one I noticed this morning.

I'm on the latest version of the OS, of Final Cut, and of iMac. 

Kind of disturbing that this has been such a persistent problem for so many of us.

Mac specs?

iMac
Big Sur 11.0.1
3.6 GHz 10-Core Intel Core i9
64 GB 2667 MHz DDR4
AMD Radeon Pro 5700 8 GB

Anything else?

That is odd. Any background processes going on at the time? How long of a timeline, how complex, what fomat/codec is the footage? Very odd behavior indeed.

Oh, yeah, are you using any third party plugins in that project's timeline.

No background processes other than background rendering sometimes, , 7 minute timeline, 2 or 3 layers with green screen, ProRes HQ UHD, and I usually use LenoFX plug-ins for text. Nothing much else going on. I try to keep the landscape tidy, so I'll turn off a lot of other programs except the ones I'm using with FCP, like Photoshop or Chrome to access images and audio files from stock sites.

I'll likely Chrome is the cause of your problems. I'd uninstall it. At least shut it down and reboot. The application floods the system and hogs everything.