As the title implies, if a player choses to pick up Curly Ferns to then relocate them elsewhere, like for example, an area with an enclosed Grottoceratops, then any Curly Fern eaten that has been placed by said player, will not be conveted into Fiddleheads.
I've tested this with a total of 27 Curly Ferns, and three different Grottoceratopses.
Only naturally generated Curly Ferns seem to ocasionally produce Fiddleheads when eaten by a Grottoceratops, which also seems to be done by a random chance as it's not always assured.

Closing the issue since it does seem to work as intended, and after leaving a Grottoceratops with a total of 43 Curly Ferns, I got 4 Fiddleheads in return after afk'ng in the area for about an hour.
So it does work!
Though it seems that the chances of the convergeance are lower than just tugging a Grottoceratops along the Primordial cave to eat naturally generated Ferns.
Test result:
An enclosed Grottoceratops will convert roughly 40~ Curly Ferns into 4 Fiddleheads.
A roaming Grottoceratops, even if guided by a lead, will consume roughly the same amount to produce an average of 15 Fiddleheads.