External hard drive ejected but still spinning and moving heads

I've got a 3.5" drive in a caddy that I use for backups. Up until a couple of weeks ago I could eject it in Windows and it would almost immediately spin down. Now, however, it's started behaving oddly.

When I eject it, after Windows has confirmed it's OK to turn it off, the drive is still spinning but also I can hear and feel the vibrations of the heads moving, almost as if the drive is still being written to or read from. This can continue for several minutes, during which the drive is definitely not visible to Windows, either as a mounted partition or even as a physical drive in Drive Management.

I don't think that this is a matter of the drive finishing any write operations that may have been in progress: I've tested it by powering it on and then ejecting it as soon as Windows allows, without me starting any write operations, and since the drive isn't used for any system functions, I don't think Windows would have started anything either. I've done a ScanDisk on the drive and CrystalDiskInfo doesn't show any SMART errors, so I'm assuming/hoping that the drive isn't faulty. I thought that perhaps some recent update to Windows (I'm on 1903, build 18362.239) might have caused a change in behaviour, but I wouldn't have thought that Windows would have any effect on the drive's behaviour after it was ejected.

The drive's a WD Green 2TB from 2013 which had previously been a main drive in my desktop and was subsequently repurposed. The caddy's an Icy Box one that I've had for about four years. A similar WD Green drive from 2015 doesn't appear to behave the same way in the same enclosure.

Should I consider this drive suspect and dispose of it? Is it safe to power it down while it's still moving the heads if I've ejected it from Windows first?

Thanks in advance.

It's not the fact that it's still spinning that I'm concerned about (even though it always used to stop spinning right away), it's the fact that it still sounds like it's reading/writing data. Even if it was still writing out of the cache (64MB), I really don't see how it could take several minutes to complete any write operation unless the drive was dying, and if it was, wouldn't SMART probably show something?

As far as I can tell, based on Windows' Resource Manager tool and ProcessMonitor, nothing is reading or writing to this drive while it's mounted, so I doubt it could have filled up any significant amount of cache before it was ejected. While I was typing this, the drive actually span down due to Windows considering it to be idle, though I could hear the heads moving right up until the moment it went to sleep. I didn't access the drive myself, nor do I have any scheduled tasks involving that drive, so I doubt that Windows would be doing anything with it other than occasionally checking to see that it was still there, and my antivirus software doesn't seem to have been actively scanning the drive either.

You say the mainboard can hold 1GB of data, which I assume isn't referring to the cache. What's this storage you're referring to?