AWS DefaultCredentialProvider set by AWS_PROFILE env var not working for terraform remote state

I have a simple Terraform root that provisions some AWS resources. It was initially set up with default local state. I use an AWS Profile to specify the target environment:

$ export AWS_PROFILE="some-aws-profile"
$ aws sts get-caller-identity
{
    "UserId": "REDACTED:REDACTED",
    "Account": "account_id",
    "Arn": "arn:aws:sts::account:assumed-role/somerolename/someusername"
}

And I can run terraform plan or terraform apply - resources get created in the target account. provider "aws" is configured with a region parameter only, all other details / creds are controlled via the AWS_PROFILE env var.

Now I am looking to move state to remote, with an S3 backend.

terraform {
  backend "s3" {
    bucket = "my-bucket-name"
    key    = "some/path/to/terraform.tfstate"
    region = "eu-west-1"
  }
}

When I run terraform init with this, an error is thrown: Error: error configuring S3 Backend: no valid credential sources for S3 Backend found. I have also tried adding profile = "some-aws-profile" into the s3 backend block, but the same still fails.

Does a terraform / backend block use a different credential provider chain? Any reason why this backend config is not able to use AWS_PROFILE implicitly from environment var, or even when profile is added?

I don't have any .credentials files that I use for auth - in my local environment, i am using aws sso login to automatically manage credentials via /cache/ subdirs in ~/.aws/sso or ~/.aws/cli - is this the part that is not compatible with backend?

edit adding in a snippet from ~/.aws/config

This is what my profile looks like:

[profile some-aws-profile]
sso_start_url = https://myhostname.awsapps.com/start/#/
sso_region = eu-west-1
sso_account_id = <actual_account_id>
sso_role_name = somerolename
region = eu-west-1
output = json

To set up auth, i use aws sso login once AWS_PROFILE is set, and I authorize the request for temporary credentials in whereever CLI stores them.

This was not working in 0.13.6 with the latest version of terraform provider aws (4.15.1).

Upgrading to TF 1.2.0 resolved this - SSO profile is used for credential loading in the S3 backend.