To me it sounds like something about the monitor and/or adapter is making the Mac think a display is connected and still powered on, thus the computer not staying in sleep mode.
I had a situation with my old laptop, a Microsoft SurfaceBook 2 with the Surface Dock, where my LG monitor connected via DisplayPort would go into low power mode, then immediately back on when the laptop was either asleep or powered off. The monitor would cycle between low-power sleep and back on, then see there's no signal and go into low-power mode, only to come right back on again. If I removed the Dock from the setup or connected the monitor via USB-C instead of DisplayPort, it would properly stay in low-power sleep mode. So something weird about the dock was causing it. I had to power the monitor OFF to stop it from cycling. It was a bit of a hassle because I need to turn it back on manually each time instead of allowing it to wake from low-power mode automatically. BUT there were no such problems with a different Dell monitor. So go figure.
Basically what I'm saying is that it may not entirely be the fault of the MacBook in this case, but a weird combo between the MacBook, that particular monitor, and that adapter.
Try changing around how it's connected and see if it persists (different USB-C adapter, different connector type - HDMI vs. DisplayPort vs. native USB-C if available, etc).