Sorry, no idea why it started to work. Had similar issue with bookmarks I added on my Mobile Brave (to the Brave Bookmarks Bar) not syncing…
Sync for me is just about the most important “feature” of a browser and its not reliable. One thing I liked about XMarks (good old Xmarks) was that you could force sync and overwrite remote bookmarks or overwrite local with remote… Brave seems to have zero options and just ASSUMES it works reliably all the time (SPOILER: IT DOES NOT!) 