There are several possible issues with grenade catching and Defuser perk.
(v16, linux)
1. Sam alone manages to catch a grenade even without Defuser perk. At least it works always at Grave diggers encounter, SlumGraveyard map. (ha sunglasses)
(2. Not sure yet in v16, but observed in v15: When one party member took Defuser perk, from that time on, I think, everybody on my team was catching grenades. Moreover close to 100% chance than 20%. Although it wasn't statistically significant number of instances, let me say that having such luck to get many catches in a row would be winning a lottery. Will re-check for v16 later on)