Credit to
Carl Murray for informing me of this
The sim has incorrect Entrainment+Truant mechanics on megas.
Sim: If a faster enemy with Truant (Durant) uses Entrainment on the Pokemon the turn it Mega Evolves, the Mega attacks normally on turn 1, then gets the "loafing around" status on turn 2.
In-game: If a faster enemy with Truant (Durant) uses Entrainment on the Pokemon the turn it Mega Evolves, the Mega will undergo the "loafing around" turn granted by Truant and will be unable to attack that turn. It can attack on turn 2.
Battle video: 8XAW-WWWW-WW3N-3EQ9
Interactions on things like Relic Song Meloetta and Darmanitan-Z haven't been tested but I'll follow up on that.