How does 十分/不十分 work？
Good question.

不满 is a single word. It means unhappy or unsatisfied. It cannot be separated into 不 + 十分 + 满. It's like you can't say "un-very-happy".

不满意 is a phrase, 不 + 满意 = not satisfied. It can be modified by 十分: 不 ＋ 十分 + 满意。