I've noticed a curious incident in my tenancy, specifically SharePoint Online and the dynamic homepage. Some sites are no longer appearing for users in their bookmark sites. Furthermore, these sites have also vanished from their recent sites.
Browsing to these site via URL confirms that they're present and appropriately permissioned. Clicking on the Follow button provides the "followed" prompt but they still do not appear within the bookmarked sites. I've tried a non-cache refresh and have even waited 24 hours just in case a timer job needed to complete. Still no luck.
Our standard desktop build is Windows 10 with this behaviour demonstratable in both Chrome and Edge. In terms of patterns, this appears to be happening to Classic sites that were created about three years ago. Not ALL of them, some older classic sites function as expected. Thus, modernising all classic sites with a modern homepage may well fix the solution but for now this is a little over kill.