Here’s a recording that shows both behaviors, both stemming from having the ‘Tab Groups Save’ flag set to Enable.
At first I just mouse over the ‘Shopping list’ folder on the Bookmarks Bar, but you can see it’s malformed with a red backfill instead of just having the folder icon.
(The other bookmarks I have on the Bar are just to occupy enough space to make the ‘>>’ button appear.)
With each click of that folder/button, it opens a tab for each bookmark it contains (it has 2). This is repeated.
Then, I show what happens when you click the ‘>>’ button, Brave crashes out.
I wonder if the Tab Groups Save feature ties into Bookmarks somehow to retain the Group across sessions, but I don’t know enough about it at this point.
This was all done in a Test profile. It does have some extensions installed but I disabled all of them before this test; and this is repeatable in my main personal profile, as well as on 2 different Windows systems at work (granted, they are both on a sync chain together).
All of my instances have been on Win10.