If this happens when the only plugin active is ACF then the condition I explained could only happen if there is something in the theme causing it. But you said you tried a different theme, so that would eliminate the theme, or should, unless you have 2 themes with the same issue.
If you have eliminated other plugins and the theme then the only other cause would have to be server specific.
Server caching like memcache could also be exacerbating the debugging problem. Depending on how long these types of caches last, not to mention your browser cache, you may not actually be seeing changes reflected when you refresh the page. When you are having issues it is important to be able to turn all such caching off so you can see what is actually happening. Again, I don’t know how to do this on the host you are using.
