Hi!
Thank you for taking your time to report bugs.
The text of the blessing is wrong. It should use the wording “Brittleness” instead of “Rendering” and should read
“Target receives 3 Stacks of 5% Brittleness on direct projectile hit. Lasts 5 s.”*
so its not the attack that you hit with that gets the rending effect, you brittle the armor of the enemy you hit. This will be fixed in an upcoming patch but text changes may take some time due to translations. We’re working on it as fast as we can.
For the Rumbler it does not have a huge bonus on the projectiles themselves since the impact damage have pretty good armor penetration but it will help your allies give more damage towards that enemy. The explosion will hit harder. It might be hard to test since the grenade will bounce off the target and it might not be hit by the main part of the blast. Best way to test this is to get a direct hit first and then quickly shoot a grenade at the feet of the minion. Then you should see a difference on the second explosion compared with a weapon without the blessing.
This buffs triggers “…on direct projectile hit” hence it does not trigger on the explosion part. With direct hit means you hit it directly with the projectile. That is not a bug.
Best regards!
Guen