Files lost from Fresco and the Cloud

I've been checking all my files as part of migrating from Sketch to Fresco. Luckily all my files from Sketch have been imported into Fresco, but in checking the rest of work, I have found that a large number of drawings from 2019/2020, that were created in Fresco have disappeared. Only one remains. They are not in the Creative Cloud, they are not in deleted files. I've checked everywhere I can think of and I definitely haven't deleted them. I have consistently paid my subscription so no issues there and I have only used 20% of my storage space. Luckily I had exported copies of all of them but it means I can't go back and work on the layers. I am concerned that this might happen to other work. Has anyone else experienced this? Is it a known problem?  I don't think it's possible to accidentally delete files and as they are not showing in the cloud I presume they are lost for ever. Any suggestions on where to look or how this could have happened would be appreciated.