My suggestion still is create a second system install and you can run SoftRAID in trial mode, which is full functioning.

To answer your question: Yes, all my enclosures are over three years old (the ThunderBay 4 units are from around 2021 or so, if memory serves).

That said, I only need my units to work. I'm not asking for anything special, just for my units to work reliably. And at present, they don't seem to—I'm dealing with constant ejections and rebuilds after every restart, which makes them impractical for daily use. There seems to be a workaround (like adjusting the RAID timeout in the settings), but that's not available in the basic functionality of the Standard version.

You know my standpoint: I hate being held hostage for a hardware purchase I've made. With every other provider—be it any of the NAS providers, Promise, Synology, Ugreen, Terramaster or QNAP—you don't need to pay ongoing fees for software maintenance just to use your hardware. Again, I'm on my way out from OWC and transitioning to NAS setups, but it would be nice to see whether that timeout change solves the problem, because at present, your hardware is not working as it should.

I've been using Apple Macs for over 30 years, and I've had many different RAIDs over those years—never have I had so many problems as with the OWC stuff. If there's any way to make a temporary exception for Pro access to test this, I'd appreciate it; otherwise, I'll have to move on sooner rather than later.

Thanks for your time.

All your enclosures are > three years old?

To try to address these:
When a disk is not showing in SoftRAID, it means macOS is not seeing it, Disk Utility (nor will "diskutil list in terminal) will not see the drive either, its simply not powering up (yet)
Disk disappearing when all the drives in an enclosure is a thunderbolt issue.
Disk disappearing, when only one disk in an enclosure, is a hardware (drive or enclosure) issue.

In both cases, SoftRAID is the messenger, telling you there is a problem.

SoftRAID cannot show you what is "missing", it cannot tell you the identifier for a disk that is not there. Only what is there.

When the disk is missing, expand each remaining disk tile and note the Serial number or SoftRAID ID of each disk. then when the 4th shows up, you can easily tell.

Blink disk light is a pretty solid off on. It should not be hard to tell if you let it go for a bit, it also vibrates the enclosure, you may be able to tell from that.

Anyway once you identify the disk that is the one that goes missing, it will be easy to tell next time, as it will not be in the drive list.

Then move it to the other end of the enclosure. When it happens again, is it the same disk, or a different one?

(I just noticed a bug in logging rebuild completed, I am adding it to our bug list)

Does the drive eventually always show up? Or does it require a disconnect/reconnect?

THe 12TB (4th drive) is missing. this is a hardware issue. Either that drive is faulty (disappearing), or that slot is faulty.
Give the disks SoftRAID disk labels. See if the same drive always disappears. Then move it to another slot and see if it is the drive or slot.
Then we know what has to be replaced.