No, the plugs are plastic on both material versions of the crossover pipes.
So, you purchased the plug caps new and installed them yourself or they were new (and likely already installed) on the aluminum crossover pipe? Reason I ask is because in my experience the design is prone to fatigue with the ridges on the plastic cap "stretching" away from the ridges on the pipe and this can lead to the plug being inserted into the pipe but not secured. The o-ring is placed on the plug in such a way that if it is not 100% fully inserted and locked then it (the o-ring) isn't completely engaged with the pipe. It's so much easier to draw a picture than to try and describe this type of stuff lol...
Its worth popping the cap off, double check your o-ring is in place and not cracked or faulty and then reinstalling. When you do get a nice big block of wood and a deadblow hammer and give it a nice soft whack to ensure its fully seated and locked.