I have a somehow related problem on my test site. It seems to be linked to the original resolution of the images that are used as Featured.
I used 2 test Featured Images:
(1) An image with original resolution greater than 660px x 300px: this is resized and cropped correctly
(2) An image with original resolution below 660px x 300px (namely 604px x 403px): this has been kept as is, with the result that it is not of the expected ratio (i.e. the ratio is different than that for image (1))
The overall effect is that the slider resizes at each image switch.