Image overlay has no animation first time but works afterwards?

The three images half way down the page are behaving strangely. When you hover over them for the first time, the semi-transparent overlay doesn’t fade in/out. It just appears and disappears suddenly. After it’s done that once however, it works fine. On that first sudden disappear, there is a ~10px section at the top which does fade out, and it looks strange.

I’m sure it’s super simple, but please could someone take a look.

Thanks