Dual class and weapon restrictions, thief skills

I noticed that Imoen - while gaining levels in mage cannot use the bow. Once her thief levels activate she can use it again.

I started a Archer and dualed him to Cleric. After regaining enough cleric levels he is unable to use any archer weapons.

Why is this different?

Also the following: After regaining the thief levels Imoen has "reset" her thief skills to almost nothing. i had 80 detect traps, now its something like 10. this looks like a bug.