Liquid Markup in Custom Fields

I have a number of forms in Zendesk using custom fields into which I am attempting to pass liquid markup values, as I've seen in a number of other community posts. However, I am not seeing the same results.

Date to date, as seen here:

Using PUT with the /api/v2/tickets/{{ticket.id}}.json endpoint.

{
  "ticket" : {
    "custom_fields": [
      {
        "id": 123,
        "value": "{{ 'today' | date: '%F' }}"
      }
    ]
  }
}

Zendesk's response:

{"error":"RecordInvalid","description":"Record validation errors","details":{"base":[{"description":"Appointment Date: is invalid","error":"InvalidValue","ticket_field_id":123,"ticket_field_type":"FieldDate"}]}}

Troubleshooting for the above:

• Sending "value": "2023-01-12" updates the field.
• The liquid value {{ 'today' | date: '%F' }} appropriately renders 2023-01-12 in a ticket comment.
• Sending the above to a text field yields the liquid markup as plain text (see below).
• This can be replicated in other custom fields, where liquid markup does not seem to be accepted even when it works appropriately elsewhere.
• Also having this issue with user field to custom field as seen in this comment.

Any help is greatly appreciated. Feeling like I'm missing something obvious.