This is what I've got for you.
Go to Settings > Accessibility > Dexterity and interaction > Assistant menu
From here, toggle the switch to "ON". This will create a hovering circle with a window with four panes inside it. If you interact with the hovering circle it will open a new window in which you have access to the back and recent apps buttons via an onscreen button.
Not sure what has disabled the hardware buttons, but this should give you access via onscreen software.
I know it's not a true fix, but a work around.