Hi Husselpot, Welcome to the Forums.
This may be related to:
Known Issue : Spawning a weapon that does not use standard ammo types (Grappler / Harpoon Gun / Plasma Cannon / etc.) with ammo set to 0 (zero) will cause it to be non-functional.
Mentioned in the 27.10 update.
Try setting the Initial ammo of the Grappler to a non-zero value.