That is pretty bizarre. I would boot in diagnostic mode to see if anything comes up. If nothing comes up, here's a couple possible reasons:
1) You disabled SIP for some reason to use an application that modifies kernel extensions (drivers) or adds its own. These extensions may be interfering with your system's sleep.
2) You have an external device connected to your system that is "soft waking" your system, allowing for things like power nap to work. The soft wake is misbehaving as a result.
3) You have an application that is running in the background that is causing problems during sleep. Older applications with deprecated functions may be at fault.
If it is none of those three things, and your diagnostic turns up nothing, I would boot in recovery mode and reinstall macOS. If that does not help, time to take it into an Apple store.