FindByCustomField returns empty data for WhatsApp-only subscribers

Hi,

We're using ManyChat API  to automate workbook delivery for our training program.

What we do:

1. Call POST /fb/subscriber/createSubscriber with whatsapp_phone to create a contact
2. If subscriber already exists, we get error: "This WhatsApp ID already exists: <phone>"
3. We then need the subscriber_id to set a custom field and add a tag

The problem: When a subscriber already exists, we try to find them using:

• GET /fb/subscriber/findByCustomField?field_id=14436572&field_value=<phone> — returns {"status":"success","data":[]}
• GET /fb/subscriber/findBySystemField?phone=<phone> — also returns empty data

Both return HTTP 200 with empty arrays, even though the subscriber exists and the custom field wa_number (id: 14436572, type: Text) contains the exact value we're searching for. We've verified this in the ManyChat UI.

These are WhatsApp-only subscribers (created through WhatsApp channel, not Messenger).

Our questions:

1. Is findByCustomField expected to work for WhatsApp-only subscribers?
2. Is there another API endpoint to find a subscriber by their WhatsApp phone number?
3. Can createSubscriber return the existing subscriber's ID instead of an error when the contact already exists?

Thank you.