terraform is confused about: 'Objects have changed outside of Terraform'

terraform plan run

$ terraform plan -var env=stage
aws_instance.prometheus: Refreshing state... [id=i-REDACTED]
aws_route53_record.prometheus: Refreshing state... [id=REDACTED_prometheus-server_A]

Note: Objects have changed outside of Terraform

Terraform detected the following changes made outside of Terraform since the last "terraform apply":


Unless you have made equivalent changes to your configuration, or ignored the relevant attributes using ignore_changes, the following plan may include
actions to undo or respond to these changes.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

No changes. Your infrastructure matches the configuration.

Your configuration already matches the changes detected above. If you'd like to update the Terraform state to match, create and apply a refresh-only plan:
  terraform apply -refresh-only
Apply plan? (y/n) y

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
(edited)

This problem was fixed by upgrading from 0.15.4 to 1.0.0. (only 0.15.5 in between)