I don't think its a bug either, it seems intended.
Anyway for my second playthrough I'll be installing a whole load of mods including 20 levels and changed XP tiers, so meh I'll do 12 sorc / 2 wizard / 2 Warlock and whatever else it allows cos why not.
Good for you, if you want to make the game easier and that's what you need to do to enjoy yourself I couldn't care less.
I am a multiplayer focused GM for a multiplayer guild so I have to think in terms of fairness for everyone in the session. You don't have that responsibility. Do what you want, nobody cares.
As demonstrated though this is a massive exploit and one that needs to be fixed ASAP. It totally violates core rules of 5E.
