As luck would have it, I decided to try closing the GDrive tab & then reloading GDrive in a new tab. Problem gone.
It wasn’t a chronic problem but has happened before, and it persisted until I did the above.
I’m pretty sure that prior to that I also had been through a BRAVE crash & restart without the problem being cured, which is why I also didn’t think of exiting & reloading the Drive page before, or that it would work even when I did think of it.
As for testing the issue in Chrome, I don’t use Chrome anymore except for auto-translate (when I need it). Once Brave has the latter, I hope to never need to use Chrome at all.