You might want to check all versal mobs because I just checked Versesys and it's the same issue here. Could be the same problem in Versafre too.

Thanks Masamune, and for checking Versesys too: you were right, it was all of Versal. Versal's monsters were set to need one very heavy hit before they'd flinch, a threshold sized for much tougher monsters than the Lv 30s ones they are here. Their flinch threshold now matches other monsters of the same level everywhere else, so Versariant, Versesys, Versafre and the rest of Versal should flinch, knock back and get pushed by Rush like any Lv 30s monster. LIVE now (game + all channels). Leaving this In Progress until someone confirms it in-world: give Rush a try on them and let us know.