I've found several wizard scrolls including protection from evil. And that scroll it is weird I cannot learn the spell. I can "prepare" it in my cleric list but not my wizard list. I guess it's that way because my spell slots are pooled between my classes.
This is simply due to the janky way that the game identifies things. Since you know it as a Cleric the game thinks you know it as a Wizard.
A similar thing happens if you multiclass 2 Divine casters and try and prepare a spell shared by both of them... The game will simply make you be unable to cast it if you prepared it in the "Wrong" class (Since it considers it "Unprepared" by the class the game is looking at for whatever reason)
If you really want, you can use Withers to respec out of Cleric, then scribe the spell. Which will permanently put it into your Wizard spell list allowing it to be prepared using Wizard spell preparations (Whether the game will let you use it is another matter entirely)
I think this makes it more viable to multi class a caster?
Kind of.
The older system, would allow multiclass casters to have more spell casts overall, albeit only lower level casts. The new system will have overall fewer spell casts, but you can upcast to make some spells stronger.
This can make some builds weaker (Like if you were multiclassing to get more uses of spells that don't benefit much, if at all, from upcasting) while other builds can be made stronger.
One of the more notable things comes from BG3 specific homebrew, in which Wizards can scribe any spell that they have the spell slot for rather than being based on their actual Wizard level. This is most commonly used to do things like dip 1 level of Wizard to scribe up high level spells to prepare and cast while still being essentially a fully leveled alternate class (Since level 11 will give you access to 6th level spells. The only thing you miss out on is the level 12 Feat)