Thanks for the additional info, guys - your comments have hit the nail on the head, BoringName.
It's been silly of me to forget about the Installer & Store versions having different folder locations and most important of all:
not adding a simple check such as whether or not the plugin will find the cache folder where it expects to (slonopot did warn me about these things not so long ago 🙈).
I see the store version's cache folder is buried deep in C:\Users\<user>\AppData\Local\Packages\50072StevenMayall.MusicBee_kcr266et74avj\LocalCache\Local\MusicBee
Goodness! But I will address these various file paths across the different MusicBee editions (with aid from karbock's guide too).
I noticed it effectively creates a copy of all your artwork in AppData\mb_RoundedArtwork\temp for all libraries, not just the one that is currently active.
I imagine that might take up a lot of space if you have a large library.
I only do this once at installation, and then delete that temp folder when MusicBee shuts down.
So it won't be a case that these dupes are created every time MusicBee opens - let alone that they occupy any storage space after the first session of use.