Same problem. I have a non-Steam account as well as a Steam account which SE allows me to put on the same master account (as two difference service accounts). For the last 5+ years I've been switching between them via the "Data Center" menu option as they intended...

Except I no longer can. Now I have to exit the game completely, launch a different shortcut (i.e. via Steam or via the ffxivboot.exe directly) and then select Data Center/account. There's no longer and fast account switching, have to close the game completely/log back in each time.

I'm sure they said it was only one way/wouldn't affect non-Steam people, but I'm struggling to find that