Are you hitting the m1 prompt when you manage to do the grab attack on a survivor? When you do the grab attack you see mouse icon for short period when you are just going to do the grab-attacks damage. You need to then hit m1 and get the "Perfectly Timed" score event in the right top of your screen. That is what the achievement is counting not normal grab-attacks.

Yes, I think the score event being bugged is what is causing it as I hit the grab attack as fast as I could in a game multiple times and the score event didn't say perfectly timed.

You missed it its finicky just tested it in custom match and got it done few times. Usually you have more time to hit if you do longer distance grab-attack. Sometimes its so briefly on screen that its hard to hit.