I've successfully identified the source of these files. It was indeed a native Mac app. (I am not aware of any method in which a web application can write to your filesystem beyond an explicit download prompt.)
The culprit app turned out to be Etcher.io AKA BalenaEtcher.
How I Found the Culprit
I discovered this by opening each of my Mac apps individually, hoping that one of them would attempt to update these files, given that they are associated with a user behavior tracking service. I fully opened each, played around a bit with each, and then fully quit each. I made sure to wait a few minutes before opening an application after closing another.
Eventually, I saw the .plist files be updated when I opened BalenaEtcher. The files were actually completely recreated upon opening the app, so the "Created" and "Modified" timestamps matched exactly when I opened it.
Additionally, I stumbled upon this GitHub issue for the actual Etcher.io project, which reaffirmed my belief they were using MixPanel. There are other discussions online regarding this background tracking, just search for "balenaetcher mixpanel".