I’m not sure what bookmark sync is. I search for bookmarks and sync. I haven’t set up any Sync functionality. So, bookmark sync is very likely set to whatever the default value is. I hope that helps, or please tell me where to see that setting.
I checked one more thing. With a normal window open and a private window open, the Bookmarks menu is perfect in both windows. The Bookmarks menu is truncated only when I close the private window, leaving only the normal window still open. In this case, I closed the private window by clicking the ‘x’ on the single tab that was open.
I did one more check, after restarting Brave. Opened normal window, then opened private window as before. This time I had sites open in 2 tabs on the private window. I closed the window, not each tab. Bookmark menu was fine in both windows until I closed the private window. Apparently, the issue is occurring in conjunction with closing the private window.
I hope that helps to pinpoint where things are misbehaving in the code. (I’m a developer myself, and I know it helps to have details)