Weird movie custom thumbnails in iTunes

I added custom artwork to many movies in my iTunes library. Recently those thumbnails started to behave weirdly. Here is a first screenshot, where areas pointed at by red arrows are inactive: clicking or right-clicking in them does nothing, whereas the area pointed at by a green arrow is responsive. Note the weird random pattern there!

There here is how it looks like when I select several of those movies:

There is a logic as only the area which was mouse-active is now highlighted but what are those extra vertical bars in the middle?

I am using iTunes 12.7.4.76 but this problem did not appear straight away after the last time I upgraded.