After extended testing of form changes (mega evolution included) it seems that every type of form change in battle both pvp and pve is broken. What has been tested is: Mega Gardevoir's Pixilate not changing moves to Fairy. Mega Charizard Y not setting up Sun, Mega Mawile not getting correct numbers.
What seems to be bugged is the entire change of form however. We've run some damage calculations and defensive mega pokemon take the damage you would expect from the non-mega version, implying that neither stats nor ability are being activated. This also works the other way around, Mega Charizard Y does thesame dmg as regular Charizard does, even tho it has a massive gap in Special Attack.
To reproduce this:
Basically run any mega and try out whether e.g. Drought or Magic Bounce work as intended.
Run damage calculations (or just do damage on thesame target before and after mega with an offensive mega) to verify that stat changes aren't being applied either.
According to previous reports it seems that this also relates to other form changes like how Ditto interacts in battle. We sadly haven't been able to test either Dynamax or Ditto/Zoroark/Mew yet but I'd guess they would have thesame issue as the issue seems to be related to changing a form in battle, and applying stats/boosts to said form.