Extended Idling

the same thing was happening on my 2012. i suspect it is because the engine runs so infrequently and often for very short periods of time.
an italian tune up cured it, and it might have been the egr, idk.
gen 4 egr is different, so i'm not sure if it is the same problem.

 

Primes are pretty sophisticated. Second guessing what the Prime does with EV while in HV mode is near impossible. especially if driver is looking closely at how efficiently the systems handle switch overs that you / we / us think could have should have bin done differently. Sure, if you really want to get a feel for how the car actually handles EVERYTHING it (the car) gets exposed to over its lifetime, (don't forget history data here), monitoring at slower than normal speed with no traffic or other driver distractions, provides insights most drivers never ever never see.
Once those stats get confirmed by both trial and error as well as getting reproduced enough times for a driver to be comfortable adding them to the base stats, its way easier to adjust them to normal and rush hours type situations.
I'm always second guessing the Primes behavior, especially when the car is doing something I think could be done more efficiently to meet() (my personal needs). That's my (problem), not the Primes !

 

This may not apply here, but when in Hybrid mode, the ICE will run occasionally to bring the charge in the traction battery up to a desired level. This is distinct from the warm-up cycle.

 

^ This. The RPM is about the same, but when idling it's, well, idle, as opposed to charging when the throttle has to open wider 'cause it's doing actual work. You can hear the deeper exhaust note.

At least that's how the older generations do it; I haven't listened to a Prime.