Testing the problem...
Changing the default to jpg, it wanted to save as png - my original default- till i changed it manually to jpg.
2nd shot auto saved to jpg as is set in the default.
Not having the same problem here-only had to manually change once, after changing default.
Then it stuck with it.
Maybe reinstall SSC or upgrade to latest if you haven't yet.
Here's the default page I assume you use-