Hi MikeC
I have not heard about a problem in Catalina yet I have not upgraded yet.
PS has a number of ways of saving files and you did not mention which one you use, no steps, and no pictures so quite difficult to narrow down and troubleshoot your problem.
If you have selected JPEG as the format for Save, Save As, Export, Export for Web etc, and you still show a different extension at the end other than .jpg, just overwrite the extension with .jpg and I bet you are good to go.
I know of only one way where the wrong extension is used (there might be more of course) and that is when I am going to save the file in a ceratin format based on the drop down, yet I click on a file name that exists already to fill in the filename with what I want. Unfortunately, it also populates with the extension of the file you clicked on which may not be the correct extension.
Since you did not share your exact steps, I gave you a generic solution which I hope works for you.
Let us know if that works or if you continue to have issues.
John Wheeler