Other users have mentioned recently that they are having similar trouble with a .bkmslf file in Dropbox.

Now that Firefox, Chrome and Safari have fairly reliable syncing (Firefox Sync, Sign in to Chrome, iCloud), and since most users have browsers on iOS or Android devices which they want their bookmarks synced to, we recommend that these services be used instead – that is, in this Help Book page, go with Alternative 1 and not Alternative 2.

You could still make a case for the (Dropbox) Alternative 2 if you don't use bookmarks on a mobile device, or if you trust Dropbox more than Mozilla, Google, or Apple.  But it does seem that Dropbox may have changed their behavior recently, and we haven't looked at it yet.  I don't think the extension is getting confused.  I think that maybe Dropbox is deleting the .bkmslf file and replacing it with a copy.  The macOS File Alias system should still allow it to be identified by path, but maybe I'm wrong.  I'll try to do some testing.

Maybe the correct answer is that we should drop Alternative 2.  What would you think of that?