Random product variations "not available"
In my case it depended on which variation I physically have first in the variation list. If the product I have first in the variation list has a price and positive stock number, then I get an Add To Cart button for all variations.

If the product I have first in the variation list does NOT have a price greater than $0 and positive stock number then for ALL variations I get "Product Not Available".

So for me physically dragging and changing the order of the variations is what changed whether I saw Add to Cart, or Product not Available - I had to have an eligible product listed first in the variation order.

The only catch is, it allows you to click the Add To Cart button on products that are not eligible for checkout, however it does not add anything to the cart or functionally mess things up.

Naturally I would prefer the Add To Cart button change dynamically depending on the status and info of that variation, but technically this works.