Is it true every single pokemon spawn on the map has the capacity to roll as alpha?

I've been hunting Alpha Emolga near wild zone 3 for the better part of 8 day/night cycles, and since then i've seen mutliple alphas and shinys along the path getting to them, but never them.

I've also noticed that they're perched precariously in such a a way on the roof that a larger model might not make sense visually.

Am I just incredibly unlucky? Is there a chance certain slots are locked from generating alphas?