Custom fields look lost after a write

Usually they are not: the answer to the write leaves them out. A write's answer lists the custom field rows as that request left them, so a field emptied in the request, and every row a customFieldValues list did not name, is missing from it while still stored. Read the record again to see what it holds.

Two cases do lose data:

• Products, product categories and people take a customFieldValues list as the complete set, and delete the rows it leaves out.
• null for a field empties it.

Writing by name avoids both: {"customFields": {"fieldName": value}} changes the fields named and nothing else, on every type that has custom fields.

See Custom fields and Sync values from another system into custom fields.

In the API reference: Edit a menu element.