I've been working with 'Advanced Custom Fields' too and had the same issue.
I did work out a resolution though. I saw in my case that
- CF: address
- CF: city
- CF: etc.
all worked perfectly, but
- CF: food
- CF: links
- CF: website, etc.
Any custom field name that is labelled alphabetically higher than 'field' was resulting in it being named 'CF: field_5229dbec8dec1', etc.
So, I appended each 'Custom Field Name' with a prefix of "a-", so I now had:
- CF: a-address
- CF: a-city
- CF: a-food
- CF: a-links
- CF: a-website
I then set up a test post with every custom field having dummy data added which I then published - this ensures your new custom fields are all added to the database.
Ultimate CSV Importer was then used to import all content as intended without any confusion.
It worked perfectly... it just took me a long time to figure out how to get there!
I hope this helps.