I figured out the problem. In the template file I was still calling “post-overview” instead of “post_overview” field. Strangely “post-overview” still worked even though I changed the custom field name. I assume this is because it didn’t overwrite the previous field but just created a new custom field with the underscore.
I’m running ACF v5.8.2 & WP v5.2.2
I’m having the same issue even when I’m using the classic editor plugin. Any solutions to this?