I don't think it's a bug. I think it's intended. I recall this being a point of discussion right before full release, with folks suspecting something like this would be the case after reading an article about Larian not wanting people multiclassing to have to wait to get spells like fireball.
Someone mentioned a sorcerer subclass (which isn't in BG3) as having full access to spell lists as an example of why they didn't think it would be unbalanced. And there were comments about being dependent on more than one attribute, of course.
Anyway, this has been the case since the game came out. I thought everyone was already aware. Again, it was even discussed before full release.
This is what was said:
"When multiclassing, the levels of your classes and subclasses that normally have access to spell slots are added together in a weighted formula, and then the overall spellcasting level is used to determine how many and which levels of spell slots you will receive."
It doesn't describe anything close to what we are discussing here. Granted I don't know what that weighted formula is but the idea that you can take a single level of Wizard and learn whatever spells you want and cast them with sorcerer spell slots is frankly laughable. It's pure nonsense.
This is an exploit. The people using it are exploiting the game. This is clearly a bug, in fact it's 3 different bugs/exploits all together.
There is no way in hell that anyone could or would have intended this. This is the worst exploit I have ever seen in this game in 3 years. Anyone using it is basically playing on Story mode.
