As the title says, Minior's substitutes seem to not work correctly, and doesn't actually create the substitute at all. You use the move and spend your turn, but there's no substitute in place, which allows it to take damage as normal. This issue is able to be replicated and happens every single time but only for Minior. Substitute was tested using the same Pokemon, same move, different Pokemon, and different move. Substitute does seem to function properly on other Pokemon so I believe it's mostly to deal with Minior's unique ability, Shields Down. Note the substitute functions incorrectly in both meteor and core form however, not just one or the other.

Linked is a video file of substitute working incorrectly on a Minior, and correctly on a Salazzle under the same circumstances.
https://drive.google.com/file/d/1TvcI1uJc-Tawni-MctYzIGB46iMXNI2A/view?usp=sharing

Hello there,
Thank you for the very in-depth report, it allowed me to confirm your issue fairly quickly. Tested in-game and compared the results to Pokemon Showdown, Minior's inability to use Substitute is definitely unintended. I'll go ahead and move this to our internal tracker so it can be fixed as soon as possible. Thanks again!