I sent an enquiry to Oxford University Press when I discovered a difference in the "citation form" used by British and American dictionaries.
This is their reply (with minor format changes)
Dear Ross,
Thank you for your query about hyphenation in British and American dictionaries.
There are no hard-and-fast rules on which words should be hyphenated, but because this is fluid, publishers tend to set up guidelines to ensure consistency within a text.
OUP's New Hart's Rules (Oxford, 2014) states:
Compound modifiers that follow a noun do not need hyphens:
the story is well known
the records are not up to date
an agreement of long standing
poetry from the nineteenth century
but a compound expression preceding the noun is generally hyphenated when it forms a unit modifying the noun:
a well-known story
up-to-date records
a long-standing agreement
nineteenth-century poetry
In Oxford dictionaries, the form without the hyphen is given as the headword, as you point out, although some expressions are always hyphenated (e.g. middle-class). It does seem to be the case that some American publishers have a preference for the hyphenated spelling as the citation form in their dictionaries. To quote Hart's Rules again:
Since hyphenation often depends on the word's or phrase's role and its position in a sentence, and because it is to an extent dependent on adopted style or personal taste, it cannot be covered fully in a dictionary.
So, it is true:
* British dictionaries list some compound adjectives as hyphenated and others open (separate words). They expect users to change all to the hyphenated form when they ARE being used before a noun.
* American dictionaries list all compound adjectives as being hyphenated. They expect users to know the hyphens should be dropped when NOT being used before a noun.
