I cannot sync bookmarks between Firefox on Windows and Firefox on Android

I have Firefox on two Windows PCs and a Pixel 8 Pro. If I make a change to my bookmarks on one PC, that change is quickly reflected on the other PC. This does not happen on my Android phone, which is signed in to the same account. I have tried logging out and in on the phone, and eventually deleted all data (not just cache) from Firefox on the phone, then uninstalled the app. I then re-installed Firefox on the phone, signed in to my Mozilla account, and set up my sync requirement including bookmarks. Firefox on the phone then installs a set of bookmarks which is several months out of date. I have repeated this process (delete data, uninstall / reinstall) several times, and signed in both with my email and by scanning a QR code on the PC. The same old set of bookmarks is synced to the phone every time. The old bookmarks which are synced to the phone are not synced to the PCs - those two devices are up to date and syncing correctly. It's almost as if there are two sets of bookmarks on the Mozilla server, one for the phone and one for the Windows PCs. I don't know where to go from here, can anyone help?