Our settings are original, theme, no music, dissolve transitions, 1080p exports. The vast majority of pics are jpgs but some projects may have a few png or others mixed in.
Did a few tests;
Shorter slideshow with only 5 pictures, original theme, 1080p: Same issue with choppy dissolves.
Original theme, 720p: Everything is fine
Ken Burns theme, 720p: Everything is fine
Ken Burns theme, 1080p: Everything is fine
So this only seems to be effecting original theme 1080p exports which unfortunately is the output we use for clients.