you are not learning the spells on lvl up but rather righting them down into your book. since you have 6th lvl slots you can prepare 6th lvl spells meaning you can learn 6th lvl spells. Pg. 114 has the rules for copying wizard spells into your spell book all it cares about is if you can prepare the spells of that lvl and on the same page when it lists preparing spells it states that you can prepare any spell that you have slots to cast.
To be fair i think that both of you are right. I´ll try to explain why as I just rechecked the PhB:
- As far as multiclassing is concerned, according to PhB if you have a char level 4 ranger/level 3 wizard you will have up to 4 level 1 spell slots, 3 level 2 spell slots and 2 level 3 spell slots, but, you obviously don´t know any spells of that level from any of those classes while leveling, since you didn´t reach the proper level.
- However, in the page 164-165 doesn´t specify the spells learnt from scrolls, so, we come back to each classes explanation for that as Derpie mentioned, and according to that, if you are a wizard and have spell slots of certain level, you can learn scrolls from that level.
- As the spell slots combine between caster levels, and as a wizard particularly, you could indeed learn from scrolls the same level spells as the level of your maximun spell slot, but you won´t learnt them "naturally" when leveling up.
It´s kind of tricky actually, but since the multiclass section doesn´t talk about scrolls, we must guide ourselves with the classes rule section.
