I came to the conclusion that there is an issue with Showdown changing abilities when Pokémon Mega Evolve. As you see in your replay and in this one here, Gardevoir retained the ability Prankster (from Trace) when it should have gotten the ability Pixilate after Mega Evolving.
I was not satisfied with this so I tested my theory using a Chlorophyll Venusaur. Breloom should have outsped Mega Venusaur but the results reinforce my theory as Mega Venusaur retained the ability Chlorophyll for 1 turn after Mega Evolving and thus allowing it to move before Breloom. However, the next turn Mega Venusaur's ability (presumably) became Thick Fat and lost the Chlorophyll speed boost. View the results for yourself here.
Hope this helps.
(Props to Kawaii Terlor for helping me test this out)