Grand-mère, tu es née avant ou après la guerre is correct, mamie is not the only correct answer
The answer could also been "Mamie, es-tu nee avant ou apres la 1ere guerre mondiale"
It marked me wrong when using es-tu
Grand-mère was not accepted! Reported Feb 2021
Still isn't fixed in April
Still not fixed in May
It didn't say 'world' war, so I only translated 'war' but apparently that was wrong? Surely Mamie could have been born before or after some other war?
There is no mention of world in the french sentence either, so your mistake was something else.
I'm marked as wrong for "quand est-ce-que tu est née "! I don't understand why.