Membership import does not display custom fields in the matching dropdown if they are used in a dedupe rule

I've got contacts imported from a csv and have a default strict dedupe rule that matches on three custom data fields.

When I go to import memberships using the same file, I find that the three custom fields are not available to match on and only appear as (match to contact) three times above the – do not import – line.

The cause seems to be that the following line (1032 of CRM/Contact/BAO/Contact.php):

CRM_Core_BAO_CustomField::getFieldsForImport($contactType, $showAll, true) );

which returns an array of fields keyed with 'custom_25', 'custom_4' etc, and cannot be searched properly by this line (849 of CRM/Member/BAO/Member.php)

$tmpContactField[trim($value)] = CRM_Utils_Array::value(trim($value),$contactFields);

which is looking for an array keyed with something like 'my_custom_field_name_25'.

I've tried all I can think of to get caches cleared etc and no amount of recreating the fields or redoing the imports has helped.