This issue did not exist in the test core for the Famicom Keyboard, but when using any Famicom BASIC version it appears the Backup Switch is permanently turned on. I am unaware of any way to turn off this switch in MiSTer, as it originally was on the cartridge (not the keyboard). I believe this was needed to keep the RAM alive between sessions, but leaving it permanently off would likely be best for MiSTer.
This issue did not exist in the test core for the Famicom Keyboard, but when using any Famicom BASIC version it appears the Backup Switch is permanently turned on. I am unaware of any way to turn off this switch in MiSTer, as it originally was on the cartridge (not the keyboard). I believe this was needed to keep the RAM alive between sessions, but leaving it permanently off would likely be best for MiSTer.