I am still on Win 10 but I very much doubt that it's a Windows issue.
To 'save as' to jpeg, don't 'merge visible layers', use 'flatten'. Does it work?
Another way, try to 'Export as new' file with jpg in the organizer.
You must have a setting in your current file which would lose some info when converting to jpegs.
"Also tried as some suggest, File, Preferences, File Handling, enable legacy save as - I don't have those options."
This workaround is only for the full Photoshop.