is concession stand popcorn kosher if the popcorn that’s being used at the time of checking has a reliable certification?
Nope.
What are the specific kashrut issues? Is popcorn considered fit for a king’s table?
Concession stands aren’t regulated by the FDA. That being said, how do you know what’s in your popcorn? You don’t.
If it is a place that only sells popcorn and and you inspect the place and find nothing but corn kernels and kosher oil then the popcorn would be fine. But this isn’t relevant 99% of the time.
We did this once years ago in Six Flags and we actually went into the facility and inspected everything and there were only 3 ingredients in existence. Kernels, salt and oil. They only made popcorn and they made all the popcorn for the park. That’s all they did. (This was 20 years ago probably)
In summary, there is no such rule that just because you see the package of kernels that everything they prepare is now kosher. That’s simply not accurate.