My S3 bucket ACL gets ignored

Previewing update (prod):
     Type                 Name                  Plan       Info
     pulumi:pulumi:Stack  mystack             
 ~   └─ aws:s3:Bucket     mybucket              update     [diff: ~grants]
 
Outputs:
  + bucketAcl : [
  +     [0]: {
          + permissions: [
          +     [0]: "WRITE"
          +     [1]: "READ"
            ]
          + type       : "Group"
          + uri        : "arn:aws:iam::432025153586:group/root"
        }
    ]
  + bucketName: "mybucket"

Resources:
    ~ 1 to update
    2 unchanged

Do you want to perform this update? yes
Updating (prod):
     Type                 Name                  Status      Info
     pulumi:pulumi:Stack  mystack              
 ~   └─ aws:s3:Bucket     mybucket              updated     [diff: ~grants]
 
Outputs:
  + bucketAcl : []

Can anyone offer a hint please?