I confirmed that my cleric with a 12 intellect was unable to MC Wizard or Artificer before having or attuning to [magicitem]Headband of Intellect[/magic item]. I'm not certain that this isn't intentional as the PHB post about multiclassing isn't 100% specific that it shouldn't be. It does seem strange that someone could simply borrow an item from another person to get the abilities that they would want to have.
Here's the post that made me aware that it was happening.
https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/64376-multiclass-with-stat-items#c3
I confirmed that my cleric with a 12 intellect was unable to MC Wizard or Artificer before having or attuning to [magicitem]Headband of Intellect[/magic item]. I'm not certain that this isn't intentional as the PHB post about multiclassing isn't 100% specific that it shouldn't be. It does seem strange that someone could simply borrow an item from another person to get the abilities that they would want to have.
Several rules are poorly written and that is why there is Sage Advice (basically the official "here is what we meant" source).
DDB does not fully prevent you from breaking the rules. Just because the character sheet allows it, does not mean the rules allow it.
It also isn't a bug when the character sheets diverge from the rules. Features that don't exist aren't broken.
In the end, always follow the rules the DM is enforcing.