Update (0425Z): all items should now be RESTORED. I have confirmed this on my own account and two other accounts from our Moderator team. We apologize for this outage. If you are still missing purchased items, please restart Microsoft Flight Simulator.
Update (0330Z): our support teams believe they have identified the cause of this issue and are currently preparing a solution. I can confirm that none of your purchased items from the Marketplace have been lost, and everything will be restored to your account.
We are currently investigating reports that players have lost access to some purchased Marketplace content. We have escalated this to the appropriate support teams and will provide an update as soon as we have more information to share.