I think the problem with reproducing framerate issues is that the game doesn't really have a benchmark.
Like, I think I'm seeing it on my system as well, which is quite different, but how do I check if it's real or my imagination?
I'm parked right now at a scene, engine running, and watching my fps fluctuate between 28 and 61 fps (live server).
The PTS version fluctuates less (15-32 when it fluctuates), but unfortunately settles on 18 fps with the same video settings when parked at approximately the same place (different time of day though), and that's concerning.
I run the live game on Ultra pretty much, and needed to dial the PTS down to "high" to get acceptable frame rate (as reported by geforce overlay).
Specs: Intel(R) Core(TM) i5-9600K CPU @ 3.70GHz , 16 GB RAM, Windows 10 64-bit, nvidia GeForce GTX 980 Ti (16 GB RAM, 461.92) 4800x900