Lead custom fields not visible

Hi all,

I hope someone can help me out here.
I am trying to add a lead to PipeDrive using the /lead endpoint and the following json:

    "title": "New Lead TEST",
    "person_id": 20256,
    "860c587186ecd1aa0c395ef9f11a686b06adfa38": "[Waarde voor Energie in verbruik]",
    "201c84faa95c148f528cb92df2545fc21cfd8def": "[Naam van de klant]",
    "8718ef431f180b92262f9647441c33aea560f0db": "[email@voorbeeld.com]",
    "e4c88a63ffa2ac21939c7d4f73d205812ab393d9": "[Plaatsnaam]",
    "a102b69d314756fef611d5555a56c4d739174be1": "[1234AB]",
    "1d067daea8d7338ccf187a048b7afec6a473150b": "[42]",
    "4438627b46d6eaa1b50ffda9c63aa6333565ea88": "[Lead afkomst informatie]",
    "825f6c5ef522dc42486c550d24d3ba87a139df6b": "[Daktype informatie]"

So I first added the contact person of our company, and assigned this lead to him.
When I asked our customer if he received the lead, he said yes, but the fields are not set:


what am i doing wrong here? I couldn’t find the solution in the docs, so I hope one of you guys can help me out. Thank you in advance!