@Halycon Styxland
This is a bug. You can copy scrolls up to your spell slot level, but you shouldn't be able to prepare spells higher than your level.
Example: If I'm Cleric 10/Druid 2, I would still have access to lvl 6 slots, but I wouldn't be able to cast any level 6 spell, only upcast. Why would a Sorcerer 11/Wizard 1 be able to cast any wizard level 6 spell when they can't even cast anything above 1? Why can't I learn fireball from leveling up, but I can from a scroll? It doesn't make any sense.
Anyway here are the PHB rules.
Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1st-level ranger spells based on your levels in the ranger class. As 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.
Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a level for which you have spell slots and if you can spare the time to decipher and copy it.
This means that a Sorcerer 11/Wizard 1 can learn up to level 6 spells from scrolls but they won't be able to cast any of them past 1.
