Aftermarket light and an aftermarket gasket That would be hard to call from here. I have a spare OEM gasket and it is closed cell foam that is quite firm. Maybe compare the new light with the old one and if they are the same shape you could replace just the gasket. Oem pn 20998779. Of course that doesn't rule out the possibility of a defective light assy.

 

Yeah, aftermarket light assemblies are prone to leak between the lens and the housing because of less expensive manufacturing processes. Thats , in part, why OEM lamps are so darn expensive. They have to go through much more rigorous standards than just DOT. Id suggest getting a replacement LED unit and try again. If the second unit fails you can try to add sealer between the gasket and paint, gasket and housing, and a bead around the outside of the housing to lens joint.
I have aftermarket lamps all around on my truck. The headlamps seem to be working pretty well after 2 years. One of my marker lamps steams up occasionally. I put the bead around the joint and that helped for a while. I think one of the bulb gaskets is a little porous. I got fancy LED tail lamps 18 months ago. One of the lamps now leaks and holds water. Literally drilled a tiny drain hole in the bottom and it is working. How long, who knows. For my CHMSL, it is working well and no leaks going on 3 years. However, it is yellowing from the UV exposure. I siliconed that to the paint on install, just to avoid a potential issue.
So aftermarket lamps are a crap shoot. That has been my experience.

 

This is one of those deals where you get what you pay for. The OEM lamp is superior in quality but is more expensive. Maybe your best move is to go back to the OEM with a new gasket. I know my first replacement faded after about a year but the second is holding up better. I also remember that I had to buy different length screws as the aftermarket lamp mount is a different dimensional size compared to OEM. In any case a thick gasket is best to use as the mounting surface isn't 100% flat and the gasket compensates for that. I wouldn't be surprised that the slight body curve causes the aftermarket lamp seam to split and fail when fastened down versus the better quality OEM version.

 

Here's the gasket you need

 

On both of my trucks, I sealed the factory brake light by removing the factory foam gasket and working foam weatherstrip (1/4" x 3/8") around the light fixture itself, and then pushing it in place and securing the screws. No silicone on my trucks. In my current truck, a former government vehicle, they actually bent the cab sheet metal to get some sort of roof light through, so I had that to push back in place, but no leaks. IMO the silicone makes a mess and will be hard to remove if you need to get access to lamps for any reason.