There is an issue with the latest Brave Beta (the Nightly had this issue for a while). If you export bookmarks to a HTML you can no longer import this HTML to Firefox for example. Furthermore bookmark sync addons like Eversync no longer are able to backup bookmarks. They act like there aren’t any bookmarks in the browser.
I can confirm that bookmark sync extensions like EverSync do not work in >= Beta builds.
EDIT ALL builds do not work
bookmark sync extensions like EverSync do not work in latest release version