The most recent update was likely Fly deploy changes for process groups and auto-scaled services which may have introduced some other changes under the hood.
Looking through the Terrform provider output (via DEBUG=1 TF_LOG=debug terraform apply) I can for example see the following redirect chain:
which then returns the Swagger docs. I don’t know the significance of this initial request for the provider though.
Unfortunately the (unsupported) Terraform provider seems have become more of a liability than a helpful shortcut.