"Error: Conflicting Parameters" on `role_arn` when `assume_role` block is defined

Under circumstances still under investigation, init fails with the following error:

[INFO] (deploy track/step/primary/us-east-1)
[INFO] (deploy track/step/primary/us-east-1) Initializing the backend...
[INFO] (deploy track/step/primary/us-east-1) Initializing modules...
[INFO] (deploy track/step/primary/us-east-1)
[INFO] (deploy track/step/primary/us-east-1) Warning: Deprecated Parameters
[INFO] (deploy track/step/primary/us-east-1)
[INFO] (deploy track/step/primary/us-east-1) on backend.tf line 2, in terraform:
[INFO] (deploy track/step/primary/us-east-1) 2: backend "s3" {
[INFO] (deploy track/step/primary/us-east-1)
[INFO] (deploy track/step/primary/us-east-1) The following parameters have been deprecated. Replace them as follows:
[INFO] (deploy track/step/primary/us-east-1) * role_arn -> assume_role.role_arn
[INFO] (deploy track/step/primary/us-east-1)
[INFO] (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1) Error: No valid credential sources found
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1) Please see https://www.terraform.io/docs/language/settings/backends/s3.html
Error: (deploy track/step/primary/us-east-1) for more information about providing credentials.
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1) Error: failed to refresh cached credentials, no EC2 IMDS role found,
Error: (deploy track/step/primary/us-east-1) operation error ec2imds: GetMetadata, failed to get API token, operation
Error: (deploy track/step/primary/us-east-1) error ec2imds: getToken, http response error StatusCode: 400, request to EC2
Error: (deploy track/step/primary/us-east-1) IMDS failed
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1)

When the following block is added to backend.tf in the step:

  assume_role = {
    role_arn = "[MY_ARN]"
  }

The error message at runtime changes to:

[INFO] (deploy track/step/primary/us-east-1)
[INFO] (deploy track/step/primary/us-east-1) Initializing the backend...
[INFO] (deploy track/step/primary/us-east-1) Initializing modules...
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1) Error: Conflicting Parameters
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1) on backend.tf line 2, in terraform:
Error: (deploy track/step/primary/us-east-1) 2: backend "s3" {
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1) The following deprecated parameters conflict with the parameter
Error: (deploy track/step/primary/us-east-1) "assume_role". Replace them as follows:
Error: (deploy track/step/primary/us-east-1) * role_arn -> assume_role.role_arn
Error: (deploy track/step/primary/us-east-1)
Error: (deploy track/step/primary/us-east-1)

It is suspected this is due to RunIAC injecting role_arn as a -backend-config=key=value command line param at runtime.

Perhaps it is not aware of an assume_role element in the backend config map and does not think a role_arn is defined?

Start at plugins/terraform/stepper.go lines 362-363 and inspect how role_arn is injected into the backend configuration. Reproduce init with the backend.tf assume_role block shown in the issue, then verify that the resulting Terraform configuration no longer reports conflicting parameters and still supports the intended role assumption.