I’m afraid I still couldn’t reproduce this issue. When I choose the wrong format, the select button is disabled. I’ve attached a screenshot for your reference.
Have you tried to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) with only ACF activated? Sometimes, WordPress plugins and themes are not coded correctly, and it can cause a conflict between each other. By deactivating them, we will know right away if there’s a conflict on your site or not. If we know which one caused the issue, we can focus on that theme/plugin alone to solve it.
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