Yes , Yes, and Yes, "tu ne m'as pas trouve" is perfectely correct. Duo is a little bit tired , for the New Year. Both of them are correct. Feminin and masculin.And the voice is a robot not a human person........
It is accepted as well. When the exercise was created it would only accept "trouvée". It has been fixed long ago.
Actually, it is not correct. That is the wrong verb tense - pluperfect in English, plus-que-parfait in French. It would be "Tu ne m'avais pas trouvée"
When the compound verb uses "avoir", the past participle (in this case, "trouvée"), agrees with the direct object of the sentence, provided the direct object appears before the verb. That would mean it is the speaker who is presumed to be female.
(It is true that the robot voice sounds female, but DL normally does not recognize that when assigning gender in spoken phrases, so either "trouvé" or "trouvée" should be allowed)
And, btw, the m' in "m'as" is short for "me" not "moi".
You have probably learned by now but the object pronoun must be placed before the verb. Tu ne m'a pas trouvé(e).