Larian broke this too - under 5e rules - which they supposedly implemented - a Wiz 1/cleric 11 should not be able to prepare (and thus cast) level 3 wizard spells. You would need a wiz 5 to do that. All you should be able to do is up-cast a spell you can prepare (cantrip/level 1 in this example) - that was another game unbalancing move on their part. It was also pointed several times during in EA. So the notion of dipping for level 1 to get haste & fireball should not work - and is way too cheap a price to pay for level 3 spells. The language around MC spell casting is not well written - so maybe Larian simply misunderstood. They basically made multiclassing jointly more powerful then the base classes - you should always lose something when multiclassing, that has always been the price for some additional versatility (which is not meant to be balance breaking, some edge cases notwithstanding).
