Sounds like you'll need to contact SE (these forums are more for players to help other players), but have you tried logging into the Mogstation which controls all the XIV related account settings? That should give you more information as to what is going on.

But no I can confirm they don't revoke service for logging into the Steam version from non-Steam, because they're actually exactly the same download (all updates comes from Square Enix directly, not Steam. The only actual difference is the code used to activate flags the account as Steam so all expansions must have a matching code).