There are some users on the official MSFS forums reporting similar behaviour with not being able to select LEGACY interaction: https://forums.flightsimulator.com/t/cannot-select-legacy-cockpit-interaction-reverts-to-lock-xbox/667061
It's not something I've ever come across before, and there aren't any solutions that I can see from a brief search. With only a small number of users reporting it, that would point towards there being something in common with the setups of those users that is causing it. It'll be worth checking that forum post above to see if you have anything in common with those users, and then replying to that post with any additional information.
Mark - Just Flight