I think mydeck is something Anki creates by default for new users. I know it looks like I'm using different accounts with different clients, but I definitely am only using one. Tried logging on and off repeatedly.
The weird thing is, there is no error and the application seems to sync stuff (it sends payload etc), but the update is not visible from other platforms/clients. It's as if Anki was syncing to somewhere else, some other account, but I definitely only use (and have only had) one, so there's no way I could be syncing my stuff to another account. I think I'll try reinstalling Anki again, deleting my account and opening a new one using a different e-mail address.