Thanks to both of you for the info, I do appreciate your responses~ @Bejaymac, I've not had any issues with DAZ products, either, but then I would expect DAZ products to work in DAZ Studio with absolutely no issues.

After many months of very careful checking of the files which I've had these experiences with, I'm almost 100% certain that it's the DAZ gremlins & not the products or files.

If anyone else finds the same, or similar, issues, please feel free to add info here~ I think it would be beneficial for both DAZ & Poser users.