I chose a picture in MEP MX Premium. Under Rotation/Mirror I chose "Straighten Horizon". Using the slider, whether I go left or right, it straightens the horizon and the photo becomes bigger to fill in the screen.
I cannot reproduce your issue on my PC. This function is workinh identically to MEP 17
On my MX installation moving the slider right or left rotates the picture but squeezes it vertically so that black lines appear top and bottom. The image appears to be zooming in horizontally as expected (as normal in MEP17) but doing the opposite vertically, requiring a reset of the pixel dimensions and AR in "size/position".
So looks like we have three different scenarios between us. Are we all on MX V 11.0.2.2?