I found a thread on 4chan (archived here
) where an anon mentions this, regarding a Swampert Mega Evolving during the Primal Groudon battle:
During the Groudon battle, I switched out Swampert, and when I brought it back into the battle, it was in its base form and unable to evolve.
On both version 1.0 and version 1.1, I Mega Evolved my Swampert on Omega Ruby during the battle, then switched out and back in, but unfortunately the glitch didn't work. Does anyone know more about this, or if it's really real? I was using a European version, but I don't know if that matters.