Confirmed. I added the formatsaveall=jpeg line to my ini file in the [start] section (which is also where the saveallpath option already exists, and that works properly), but it still saves in png format. I also tried spelling it "jpg" and I tried changing the "defaultformat" under [convert] from png to jpeg (since it's the only other thing in my ini file which is set to png), but still no results.
I'm using xnview 1.99.1.
Oh the feuhrer, oh the feuhrer, oh the feuhrer's nipples bonk!