Larian kept it true to tabletop. the PHB stuff is little misleading. once you multiclass casters you throw out the wizard and sorcerer slots all together they go by the chart on page 165 of the PHB. Secondly you can learn spells of any level you can prepare since you have 6th lvl spell slots you can prepare 6th lvl spells. this is working as intended but i can see why some may think its a bug, its been this way since 5es launch what really changes the power of play is no hard limit on lvld spells a turn. in 5e which theyve based most of the design around if you quicken spell fireball you cant cast another spell that turn unless you have multiple action.
PFFT! You need to maybe read the actual book. What a load of nonsense. We already quoted the relevant paragraphs that state explicitly that this is not at ok.
If you read page 165 it 100% backs me up on what I just said. The chart on page 165 is designed to *limit* the number of spells a player can have. Since you get more spells slots at a lower level than you get at higher level - it prevents a character from having too many spell slots because they have multiple casting classes.
