Tell me if I understood correctly - client pressed too long, that’s why he got subtitle offset instead of skip forward?
Solution to this would either be tapping quicker or turning off the new keymap by going to Settings>System>Input>Peripherals>OSMC RF Remote>Don’t use the custom keymap for this device> [enable] ?
There is a significant window between what is a short and long press so getting that to happen by accident should be difficult. You could turn off the custom map but that will not remove long-press support so if they hold that key down it will activate fast forward instead (Kodi default mapping). If you wanted to make it so the long-press did not perform an action at all you would have to manually add a keymap and noop the long-press action so only the short press is registered.