I know on the old Intermec CK71 we always had to use an older wpctsc.exe that automatically send windows a flag called HI_RES_AWARE and launched programs automatically in full screen mode.
Not sure if this is the case with your Zebra Client. But then again, you said it worked in older Windows and is a new issue during Windows 2019 only. ( @jgiese.wci ) in this case it has to be Windows related, no?
If you have tried the stuff in the previous posts from Microsoft, try changing your Scaling now back to 100% and don’t zoom it in. It looks like, now you are too far zoomed in, go back a notch in DPI Settings.