terraform-provider-google: 2.20.0 breaks google_compute_region_backend_service with "backend.0.group": required field is not set

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Terraform Version

Terraform v0.11.14

• provider.google v2.20.0
• provider.google-beta v2.20.0
• provider.template v2.1.2

Affected Resource(s)

• google_compute_region_backend_service

Terraform Configuration Files

variable "backends" {
  type = "list"
  default = [
    {
      group = "${local.instance_group[0]}"
    },
    {
      group = "${local.instance_group[1]}"
    },
    {
      group = "${local.instance_group[2]}"
    },
  ]
}

resource "google_compute_region_backend_service" "internal" {
  name                  = "${local.backend_service_name}"
  project               = "${local.project_id}"
  region                = "${var.region}"
  health_checks         = ["${local.health_checks}"]
  load_balancing_scheme = "INTERNAL"
  session_affinity      = "${var.session_affinity}"
  protocol              = "${var.ip_protocol}"
  backend               = ["${var.backends}"]
}

Debug Output

https://gist.github.com/korjik/70eb44a79671cb4d686ae3c0a0ede1c2

Expected Behavior

terraform plan works fine

Actual Behavior

Terrafrom plan breaks with:

google_compute_region_backend_service.internal: “backend.0.group”: required field is not set

Steps to Reproduce

1. Use Google provider 2.19.0 and run terraform plan over a resource google_compute_region_backend_service - all works fine
2. Change provider to 2.20.0 and the error will be thrown

b/309764960