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.
