It's not massively unreasonable - IRL grenades don't do AoE damage, they throw out a cloud of metal chunks called shrapnel
ideally shrapnel dispersion is uniform to maximise the chance of hitting people close by, but this is not a certainty
there are instances of grenades detonating quite close to people and they're just lucky enough that the shrapnel completely misses them
so, being close to a grenade and only getting hit in the hand is plausible
what's up for debate is whether this is desirable game design
knowing torn it could be any one of:
- an unannounced change
- a bugfix for a bug players have been relying on for a decade
- a new bug that was introduced and not caught in testing