Preview error while cutting

So basically this is what happens when I try editing some photos in Preview.

It only occurs after I cut them (cmd+k) - and sometimes not even until after I save them.

In most or all cases I have edited them in other ways - changing color or contrast.

After I save I can not return them to the original condition - can't delete the darker part.

After saving, the images appear like this in any other program I use to view them. Also when uploading them on the internet.

I haven't noticed this glitch when editing in any other program I use for photo-editing. Only Preview.