I had the exact same problem. I was getting ACF names like “field_5229dbec8dec1” and I couldn’t figure out how to get the proper names to appear even though that field had been named “food” and dummy data had been added to every custom field and published (so Elliot’s answer above is incorrect).
It took a while, but I realised that any custom field that I had named with any letter after ‘f’ was being replaced by the default meta key name.
So… I renamed every “custom field name” (not the label) with a prefix of “a-“.
I was now able to import my custom fields as intended, albeit with an ‘a-‘ prefix.
I hope this helps as a workaround for anyone with similar problems.
