Hello,
I've been using PF3 with MSFS 2020, and for the most part, it has worked very well.
However, during an IFR flight (between KSDF & KFFO), with a cruising altitude of 15,000 feet, I kept getting an ATC instruction to maintain 15,000 feet. I was already at 15,000 feet and my barometer was set correctly (pressing the B key).
I kept getting the nag from ATC until I set the barometer to STD (29.92). Once done, the nagging stopped.
NOTE: I am using the latest MSFS 2020 beta, which is working very well for me.
Of course, the transition altitude is 18,000 in the US. I also checked and set Options 1, Trans ALT & Alt Baro Calls settings correctly by making sure the TA is 18000 (and Alt Baro pressure in Millibars is UNCHECKED).
It appears there is an issue with PF3 where I have to set STD barometer below the Transition altitude to stop PF3 from nagging me to climb.