Wait omg I think I found it, it's the vol. 4 cover. It's soooo not how I remember it though lol but I honestly think it's similar enough that there may not be another version anyways ╮(￣▽￣)╭
https://iweb11.mangapicgallery.com/r/newpiclink/blue_sky_complex/21/cc738f35bdffb44979cc553d49326421.jpeg
There’s probably a version that was cleaned somewhere online, but yep this was definitely the thumbnail image for a while!
The Spanish version looks slightly different, so it was maybe. That one you saw - https://x.com/mwediciones/status/1648707560357130242
Guys I'm trying to look for this one cover that was used for the thumbnail of this back in like late 2019-early 2020, it had kind of a dark blue background and basically showed Natsuki topless kind of bent over, just kind of sitting on top of Chika while facing him. Please help me out guys I can't find it anywhere ┗( T﹏T )┛