> especially if the laptop is hot and goes into my backpack directly.
I'd found MANY times that simply closing one of the more recent MacBooks (2019?) would not actually put it to sleep. I would close it, put it in my bag, then... hours later, find it was overly hot, still running, perhaps with 10% battery left. 2008 MB, 2012 MBP, 2015 MBP - never ever had this problem. I'm sure I overheated my 2019 MBP far more often than it was designed for.
Back in the day, when I was working at a PC repair shop, you'd see every kind of laptop come in. People would bring in Mac laptops, and then just open the lid up on the service desk and it would wake up immediately like it was still running! After all my experience with Windows PCs, I would be shocked that people would just shut the lid and put them away.
We all knew that you'd never just suspend or sleep a PC laptop and put it in your bag. It would always wake up, or not go completely to sleep, and you'd end up with a toasted laptop.
Meanwhile, every single Mac brought in had no trouble sleeping. They never overheated in a bag. The Mac owners never powered them off, always just shut the lid and put them in the bag. I'd never seen one overheat in a bag. Tons of Macs would come across the service desk, and never one overheated. Nobody ever shut them down to transport them either. They would reliably suspend / sleep every time you shut the lid.
Lately though, and I don't even work in a repair shop, I see Macs overheating in in other people's bags. I hear the fans soaring on them constantly. Mine doesn't suspend / resume properly either. They really aren't built as well as they used to be, IMO. I don't trust it to just sleep properly when I shut the lid, so it always gets powered off before it gets put away in a bag.
I think one thing I did sometimes would precede the 'no sleep Mac' thing. I'd have an external monitor connected, then disconnect it. Looking back, I did notice that often disconnecting a monitor would mean... it might take 20+ seconds for the OS to be 'normal' again. If I waited for a minute after removing a monitor, the sleep would seem to work. Disconnecting at office and putting in the bag... it never really went back to 'one screen' mode, and I presume continued to think an external monitor was connected.
I can't say that's behind all the events, but thinking back on it more, I know that was a repeating scenario.
I have had this issue on my previous 2015 MBP, and haven't had it on the 2018 device I now use, for what it's worth. The 2015 would also wake up from sleep (presumably for updates?) while in my laptop bag, so I resorted to doing a full shut down at the end of a work day.
Do you use one of those apps that keeps your screen open? Caffeine.app and the like? Some of those used to be notorious for keeping the laptop awake when the lid was closed.