Sorry for not getting back to you sooner. Thanks for sharing this with us. It shouldn't happen.
What files disappeared? Are they new or created in the previous version of AX? Did you refresh the page and see if you can access those files?
You can transfer your assets to the current version if they were made in the previous version. They will keep the original edit: • Go to the prior version here : https://adobe.ly/3HRkZQR • Click Move file on the project you want to migrate • Click Convert for Desktop • The conversion takes place
You will land in the editor for the new version of Express of the project you move. To see your projects from the old version on the new one, go to "Your Stuff" and click on the file name "Originals".
If your files were created on the new version, please share a screenshot as we can see what you're experiencing.