Is the answer incorrect?

Question

"Vous ________ à quatre heures?"Will you have left at 4 o'clock?(HINT: "you" is an all female group here)I chose "serez partiées" but the result says the right answer is "serez partiés"

Accepted Answer

Bonjour Nil,
I have checked the possible answers in this Kwiz and the correct answer is ‘serez parties’. However, ‘serez partiés’ is an answer considered as nearly correct.
The past participle of ‘partir’ is ‘parti’ and therefore when conjugated with 'être' and a feminine subject it only takes one -e : ‘partie’ (feminine singular) / ‘parties’ (feminine plural)
I hope this is helpful.
Bonne journée!

Answer

«Vous serez parties à quatre heures ?» Not sure why the 'é', as the past participle is parti, the feminine partie, feminine plural parties.