I did some more testing, if I create a new save in 2.0.20 I can see the save, but it was not in the same folder as the other saves so I searched for the save in the OS and found it located in ~/snap/steam/common/.factorio/saves I moved the saves from the original folder to this one and now I can see ...

I am also having this problem, I can't see my Space Age saves in game after steam updated Factorio to 2.0.20, and can only see my pre-Space Age (1.1.11) saves.
I can see all the saves in the ~/.factorio/saves folder, and I don't have any saves in ~/snap/steam/common/.factorio/saves folder like op.