Probably because you need to accept the loss and take appropriate action after the update. It was a relatively common side effect of updating to V68. See https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html Downgrading to an old version is no longer supported. Doing so can actually lead to permanent data-loss. What you have seen so far is not I would say. But may become so if you keep going back.