Hi,
The behavior you’re observing could be due to the way SCOM 2019 handles disk space monitoring. In SCOM, the “Logical Disk Free Space Monitor” treats system disks (typically the C: drive) differently than other disks.
Kevin Holman's Blog provides a detailed explanation of how Logical Disk free space monitoring works in SCOM. It mentions that by default, the monitor will only generate alerts when the monitor is in a Critical health state, and Warnings are for showing state/health only. It also explains that the monitor treats SYSTEM disks differently than other disks and that both the Free Space in MB threshold and the Percentage free disk space threshold must be breached for the monitor to change state.