Continuing the discussion from Margins not responding for layout:
I have looked into this now, & discovered a bug. When setting-up the print in Inches, the margins are still processed in mm! As you can see in the picture, what the Page Setup dialog clearly shows as inches, the actual Print Preview (behind it) shows as mm! (& prints accordingly.)
The 0.39in marks are auto-generated from the default 10.00mm, the 5.00in is me proving the issue. This was tested with the 6/2020 appimage in Ubuntu 18, but had been occurring previously as well.