Has anyone figured this out?
Sure, well known problem... there are web pages on the web dedicated to this issue. Google can help to find them. I have been solving sleep issues may be every second year over teh last 10 years. I have whole notebook page with notes and instructions, how to figure this out...
This page seems relatively useful for novices:
https://www.howtogeek.com/302942/how-to-figure-out-whats-preventing-your-mac-from-sleeping/
In short - some application is running preventing sleep or you my have hardware issue. Applications can prevent system from sleeping - and you REALLY want them to be able to do it - for example if you are playing 3 hours long movie, you do not want system to fall asleep every 15 minutes or so. So any app which can play some content must be able to prevent sleep... There may also be problematic/hanging apps, drivers, background processes, or even corrupted power options. For what we know, you may have it in the background running bitcoin miner, options are nearly infinite.
It is also possible that display close sensor, which should be indicating that the display is closed, may not be working (for example of hardware issue causing this). Some Bluetooth devices (even some mice) can prevent sleep, when connected.
Basically, there are way too many possibilities for this behavior and no simple answer. Rebooting may help if it is app which hanged. That is easy to try. If that does not fix it, follow web instructions and see what you can find. Finding culprit will take time, could be hardware or software.
Powering off will surely solve the issue, but takes longer to restart (would you care?). May be easiest solution, I am afraid...