In my case, I've discovered that Devonthink (document/notes management app) is responsible. I've been meaning to file a bug report about it.
I'm surprised that Apple's power management doesn't have an alert for this. Surely an app that causes my Mac to become glowing hot while sitting in my backpack, not to mention slowly running out of battery, is a pretty important thing to intercept. Meanwhile, I keep being asked if Chrome should be allowed to find devices on my network, which doesn't seem nearly as important.
I'm more surprised that any application can prevent sleep _when you close the lid_.
I can understand the utility behind something like stopping sleep via timeout so a media player can tell the system "hey, they're watching a movie don't turn off even if they don't touch you for a bit".
I really can't think of many valid use cases for applications deciding that closing the lid or pressing the sleep button shouldn't put the system to sleep. Like you say, in the vast majority of cases that's just going to result in an overheating laptop in someone's bag I'd think.
Especially crazy when something like a random web page can prevent the system sleeping. Laptop won't turn off... which of my 70 tabs is it?!
Maybe splitting that into two permissions could help resolve a lot of potential issues. Sure, let lots of things disable the sleep via timeout... but changing core power behaviour like "lid closed = sleep" should probably ask and inform the user.
Yes Apple build quality is high, but it's not perfect. It's an iron law of electronics that heat shortens lifespan, so taking a large surface that is used for heat dissipation and putting your screen directly on top... I wouldn't do that with mine but you do you.