I am also seeing occasional not ejected properly notifications. I've been tracking this issue since mid April.
I have two OWC Ministack STX units used as Time Machine drives. Each has a 14TB hard drive. One also has a 4TB SSD. The Ministack STX units were new in mid December 2021.
Sometimes the disks remount spontaneously, sometimes I need to restart to remount them.
Sometimes only one disk unmounts, sometimes both do, and rarely the SSD also unmounts.
Sometimes one or more will unmount twice in quick succession.
Once a disk unmounted as I was using the Mac. This happened while the other disk was active with Time Machine.
Once 9 days passed with no not ejected properly notifications, but otherwise it happens every few days.
I have tested pretty much everything:
Put hard disks to sleep when possible — checked, unchecked and using Terminal: sudo pmset -a disksleep [various settings]
Three different Thunderbolt 4 cables — Apple Thunderbolt 4 Pro Cable (3 m), Apple Thunderbolt 4 Pro Cable (1.8 m) and OWC's Thunderbolt 4 cables supplied with the Ministack STX units to Mac. OWC's Thunderbolt 4 cable between the units
Two Macs — 24" M1 iMac (off the shelf), Mac Studio (near maxed out) starting in mid May
Switched order of Ministack STX units in the Thunderbolt chain
Using Time Machine Back Up Automatically or using Backup Scheduler
Set Reduced Security on the M1 iMac as suggested by OWC (I left the Mac Studio at default Full Security)
Change security settings on the startup disk of a Mac with Apple silicon - Apple Support
Disk Utility in Recovery Mode on one of the 14T drives overnight. No problems found.
The M1 iMac has two OWC Pro Mini bus-powered drives for Time Machine and they never unmount spontaneously.
I have searched, but have found no app that can log drive mounts and unmounts.
I have posted the issue (briefly) to Apple Feedback.