Noticed an issue with SP Designer and dataviews within classic pages on 04/27/2020 after the MS removal of the ability to create custom list forms via SPDesigner.
Now when editing any classic SharePoint page within SPDesigner that has a dataview on it, the dataview gets its data source ripped out and if page is saved now the dataview will no longer work. You can manually add the datasource back into the dataview but if you come back to the page in SPDesigner it is missing again.
See screenshot below of where the datasource is removed when viewing on the page.
Have confirmed this behavior on multiple tenants with various admin user accounts. Tried the usual clearing of cache and such to confirm not a local issue as well. Seems to possibly be related to the recent SPO update to remove SharePoint Designer's ability to create custom forms in your environment.