I pulled "Lightning, Lone Commando/Isshin, Two Heavens as One" and thought it would fit well in an in-progress Mardu deck I've been working on. I go to add it and can't find it. I back out and try veiwing my collection (via making a new deck) and STILL can't find it. I notice that the new deck veiw tried to default to Alchemy so I swapped it to Standard. Wasn't there (I DID find it in Limited and Timeless, so I did confirm I have it collected). I tried searching for similar cards in Standard like "Shantoto's Coersion/Diabolic Intent" and "Warrior of Light/Jodah, the Unifier" and found those just fine even though they don't appear in Alchemy either.

My conclusion is that either "Lightning, Lone Commando" is bugged and marked as invalid for Standard when it should be, or is Intentionally invalid for Standard without explanation or clear reason. It's a bit frustrating that I opened a pack and got a bonus rare out of it only for it to be basically unusable.