Hi there. We work with a team who has a connected folder on dropbox. I just updated to Tahoe 26.4.1 on mac I lost about 35% of my fonts the first time i opened Fontbase. Nothing has changed in my folder either.
Are the fonts still present in the actual folder? If so, have you tried to just pick a different root folder, and then re-pick the current one?
Also, make sure you are not keeping any fonts or folders inside the Providers folder, they will be ignored.