Grenade has exactly the same cooldown no matter if you are radiant or amplified.
Perk description reads that grenade recharges faster when amplified or radiant, but this is not the case.
It is the same bug it had when it first came out a long time ago.
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Hey there, Thanks for reaching out to us. We understand the Shieldcrush artifact mod isn't increasing grenade regeneration. We're sorry to hear of this, as we can imagine the impact of such an issue. Should you continue to experience this, then please consider recording a short video of what you experience. If you record a video, you can upload it to a site like Youtube or Imgur, and link us to it here. We'll then investigate.