TL;DR at bottom

I recently resealed the flue joint on my Direct Vent Gas Fireplace with 600F Red silicone sealant. I noted that the same material was used to affix the fiberglass gasket around the glass front of the fireplace and concluded that the red silicone would be safe to use on the flue gasket.

Over the summer I had disconnected the flue vent from the fireplace so I could attach the AC exhaust to it, there were no windows nearby in my tiny apartment to properly run the AC exhaust. I noticed a fiberglass ring type gasket that was glued to both the top of the fireplace and the underside of the flue joint on the vent pipe with red silicone sealant. The gasket basically disintegrated when I removed the pipe, thus the reason for needing to reseal the joint now.

I had previously used 2000F stove cement on the joint but it cracked and flaked off quickly and I was worried about leakage. I applied the red silicone both as a ring between the flue joint and the top of the fireplace and as a caulk line around the edges of the joint. The first time I turned on the fireplace everything seemed fine, though I didn't run it for too long. The second time, after about 10 minutes or so I noticed a growing smell of hot plastic, I looked inside the top with a flashlight and saw small wisps of smoke rising from the exterior of the flue pipe.

The top of the fireplace itself was sizzling hot to the touch, instantly vapourizing the layer of saliva I'd put on my finger to test it. However the silicone was fine, not soft or bubbling, and cool enough to touch for a few seconds. I even removed a small piece of cured silicone and heated it up over my stove to compare the odour, I noticed that the silicone did not release any odour until it went past its temperature threshold and began to turn brown, then it smelled more like burning pitch than plastic.

I'm fairly confident that it's not the silicone that's causing the odour and perhaps is just some dust or lint from synthetic fabrics that's gathered on my fireplace while it was off for 7 months. Though I vacuumed the top when I was doing its fall cleaning before turning it back on, there seems to be a lot of really stuck on fine particulate still there.

TL;DR
Used 600F red silicone sealant to seal joint between flue pipe and gas fireplace, hot plastic smell present when first run, suspect it was the sealant but testing seemed to suggest that the odour was not from the sealant.

Was the 600F silicone the correct choice for the flue joint? Please advise, thanks!

Photo of my sealant job after the fireplace was run for 15 minutes is attached - I may have been a bit heavy handed with the caulking gun, it was an awkward space to get the gun into and ended up just having to squeeze clumps on my finger and spread it that way.

Thanks!