CNCjs just displays whatever value was reported by GRBL live feedback, and GRBL happens to default to mm units for that status report, regardess (I think) of the G20/G21 codes being executed.
You can change this behavior by setting $13 GRBL parameter (“Report Inches”) to 1, and then the feedrate in inches per min will show up in that field.
Grbl parameters are stored in the EEPROM — you interact with them by sending commands to change them, so for instance in Carbide Motion you would go to the MDI tab and send
Grbl has an option for reporting in either measurement system — where else would that preference be?
The question rather would be why CNCjs doesn’t accept the reports in whatever units they might be in, and then convert the measurements as appropriate.
I think you want them to stay with the machine. You’re trying different senders, someday you may switch computers. It’s a good thing the parameters are stored on the controller.