I hope this is just a bug. But what I am afraid is they are trying to implement new system : only one account allowed per device. Which means you have to use different device to play different account
Update: ok, I just read the terms of use ingame (main menu > legal > terms of use, point 5), there are actual rules for accounts or ID, its stated that you shouldnt have multiple IDs, and sadly i think this is actually a new feature to prevent switching account on one device.. and the fact that there are no announcement for this, yep, goodbye for my side account i guess