Sealing Off External Chase Vent

Hello!

My house built in 1992 came with a zero clearance gas burning superior BRF-3875 which was replaced with a Heat & Glo Supreme i30 gas insert.

We are still experiencing drafts from the fireplace despite the insert.

There is a vent on the outside of the fireplace chase. The vent is grated and comes with a cover. I can tell the duct goes vertical but does not penetrate the firestop (I cannot see it from the attic), and I am not sure where it terminates. The insert installers stuffed fiberglass insulation in the duct, but its certainly not air sealed. My concern with sealing this is that it might provide cold air ventilation for the firebox to prevent it from getting too hot and combusting nearby materials. Or was it used for combustion air for the previous fireplace and can be sealed off to help with drafts?

If it can be sealed, how would I go about that?

Thanks so much!