All external SSDs overheating - depending on content!

I have two identical UGREEN CM850 USB4 SSD enclosures which have PWM fans that are temperature-triggered. I have four NVMe SSDs, a mixture of WD Blue SN550 and Crucial P3, but all PCIe Gen3 x4. I use the SSDs for virtual instruments, sample libraries, VM images and the like, and generally speaking performance is very good. I connect the enclosures to my MacBook Pro using the supplied cables, sometime directly and sometimes via a CalDigit Elements Thunderbolt 4 hub. All the SSDs are 'exempt' from Spotlight scanning by virtue of adding them to the 'Privacy' list for Spotlight.

My issue is quite bizarre (or so it seems to me!). If I place any of the SSDs under load, for example copying large files, in either enclosure, eventually the fan will kick in and eventually the SSD will cool back down once the operation is complete. But there is one set of content that, no matter which SSD I copy it to, or which enclosure I put the SSD in, or which connection I use (direct or via hub), the enclosure fan will kick in within a minute or two of connecting and keep running continuously, even though I haven't initiated any action that would read the SSD. The fan keeps running even if I unmount the volume!

Occasionally, the volume is also disconnected by the OS or, at least, not by me. The 'Disk Not Ejected Properly' notification is displayed suggesting I should 'Eject <volume> before disconnecting or turning it off'.

The volume of content is: a couple of large Mac applications (specifically games) that I'm not running, a media collection (videos and music that's not playing), a couple of Parallels VM images (not running) and a VMWare Fusion VM image (again, not running).

Which ever SSD I copy this collection of files to doesn't matter. Put the SSD in either of the enclosures, connect it to the Mac and it heats up very quickly and doesn't cool down. This doesn't happen with any other volume content on any of the other SSDs in either of the enclosures.

Are there any macOS users with external SSDs that have seen this kind of behaviour? I'm stumped! Big thanks for any help and advice you can offer.