Never noticed that before either. I do know about a few other cards with this error, like the Misty’s Seel, but I didn’t knew about the No Rarity Vulpix having this error.
Here’s another fun fact about this artwork: this Vulpix and the vending machine Mew have the same background (first discovered by @muk and shared in this thread), and the EX Holon Phantoms Regice also has it as foreground (discovered by @rattledkarp and shared here):
Here the original source of that back-/foreground from the Sozaijiten website.
A lot of the early TCG artwork backgrounds, especially for Ken Sugimori artworks, were taken from those stock images. More info about that in this thread (and originally this Twitter post).
Great collection btw. Out of curiosity, what made this artwork your favorite card? And is Vulpix also your favorite Pokémon in general?
Best of luck completing it!
Greetz,
Quuador