I'm not convinced that it can happen so rapidly. My suspicion is that the bugs already had bacteria or fungi in their digestive systems that were able to break down plastic, but that capability was latent in the fungi. We only notice it now because it's a helpful ability. Bringing out a latent ability isn't really evolution.

That's unlikely, considering these kinds of plastics simply did not exist before. Famously, scientists were able to study in detail the evolution of the brand new capability for a group of microbes living in waste water from a factory to metabolize nylon, a completely artificial polymer with no natural analogue. Moreover, scientists were inspired by this to try replicating these conditions in a lab, with a brand new strain of bacteria. I think it took a few decades, but they eventually found that that strain, too, evolved the capability to metabolize nylon, despite the required enzymes definitely not being present ahead of time. Microbial evolution proceeds at blinding speed, so maybe you could argue this doesn't apply to insects, but insets also have pretty short gestation cycles and at least some species are clearly experiencing dramatic selective pressure due to human activity, so I don't think it's at all inconceivable.