test.j2

terraform {
  required_providers {
    {{ tf_provider['name'] }} = {
      source  = "{{ tf_provider['source'] }}"
      version = "{{ tf_provider['version'] }}"
    }
  }
}

test.yml

- hosts: localhost
  gather_facts: false
  tasks:
    - template:
        src: test.j2
        dest: out
      vars:
        tf_provider:
          name: "null"
          source: "hashicorp/null"
          version: ">=2.0.0, < 3.0.0"

If you replace lines 8-11 of test.yml by:

        tf_provider_src:
          name: "null"
          source: "hashicorp/null"
          version: ">=2.0.0, < 3.0.0"
        tf_provider: "{{ tf_provider_src }}"

then it also crashes.

Or simply use this template:

{{ a.name }}

and this playbook:

- hosts: localhost
  gather_facts: false
  tasks:
    - template:
        src: test.j2
        dest: out
      vars:
        a: "{{ b }}"
        b:
          name: foo