How did the Latin past participle suffix -atus develop into modern French -é?
Considering the two following examples: modern French état ("state; status") and été ("been"). Both derives ultimately from the Latin past participle status. But while the former, as a noun, retains a somewhat faithful representation of the root word, the latter deviates for some reason. Compare cognates in Italian, Portuguese and Spanish which have perfect parallels: Italian stato, Portuguese and Spanish estado are the single form of both the noun and past participle.
On another note, considering the suffix -té of the unrelated noun été ("summer") which is on the other hand, regular, as it derives from the Latin suffix -tas.
Basically, the most straightforward routes would be Latin -atus > French -at, -as > -é. Yet the past participle ending -é doesn't follow these routes. Why?