@ranqt there's no such thing.
Given that you're using an emulator to catch Mew: emulators sometimes have hiccups during PIDIV generation, which can cause legality issues.
[such hiccups are problematic, because actual hardware generation won't lead to those]
Your Mew is illegal due to IVs. (How the RNG works, is that PID is called first, then the IVs)
I've just tested it with a single state save, and both Mew, transformed or not transformed, PID and IVs are exactly the same.
If you don't believe me, try bumping into the Mew, then press A (to trigger encounter), and immediately state save.
Then capture on state where it transformed, and capture another state where it didn't transform.
You're gonna need to save the game after both states, and export both saves. Then compare the Mew on both saves.