Predictive Routing 400 Error on Skill Assignment

How come this setting causes a 400 error when pushing skill assignments via Terraform?

Context:
Environment: AU-1 BYOC
Provider: genesyscloud v1.22.0
Resource: genesyscloud_routing_skill

Pushing new skill config fails with “Invalid skill assignment”. JSON payload looks valid. No changes in API schema recently.

resource "genesyscloud_routing_skill" "test_skill" {
 name = "Test Skill"
 enabled = true
 description = "Test"
}