No, they didn't fix that. The Ghastly line is still weak to psychic. What they did change was that in Gen I, Psychics were immune to Ghost Types.It wasn't a bug. It was the fact that all three of the Ghosts in the R/B/Y series were also Poison types, which made that type effectiveness cancel itself out.
Thankfully, they fixed this in Gen. 2 from what I remember. I'll find out soon enough as I'm almost "done" with the color series. (And once I buy a new battery...)