We confirmed that there's a bug with Salt Grinder specifically. Well, it's not quite a bug but we're gonna update it regardless. Salt Grinder is a HitGrab, as opposed to a command grab, so it needs to hit the opponent first before executing the grab, hence why it fails against Unflinching. Regardless, to make things simple for everyone, we try to have all grabs work against Unflinching.
I'll see if we can fit it in for our next update.