I've tanked this boss for a while now and I've always noticed that the telegraph for his heavy attack is pretty lengthy but somewhere in the telegraph you, as the tank, are unable to block it. Your character just stands there and takes the hit and is knocked back. The telegraph can be difficult to see amidst player and boss effects (blue fire or player ults).
I always thought this was odd since other bosses and monsters you can get your block up just before the blow lands and deflect the damage. For whatever reason, with High Kinlord Rilis, if you don't block when he starts to wind up for that heavy attack you just can't block it at all. This feels like a bug to me given this pretty much doesn't happen anywhere else in the game.
I play on PC-NA and this has been a weird quirk I've just ignored for over a year, year and half (?), probably...
Just Call Me Stalwart
PC NA
Tank First, Healer Second, DPS for Fun