When unpinning a grenade, dropping it on the floor and picking it back up, the option "Pin" does not show anymore.
In this ticket you confirmed being able to repin grenades as a feature in part of tripwires "remembering" how much of the fuse was left.
See the bottom left of the video. At first it shows "Unpin". Once unpinned, it shows "Pin". Once dropped and picked back up, the option does not show anymore.
This is working as intended, I hope. Because in real life putting a pin back in doesn't stop the fuse that has been lit by releasing the grenade from your hands.