YAML starting three dashes not preserved during encryption

The https://yaml.org/spec/1.2.2/#22-structures says

YAML uses three dashes (“---”) to separate directives from document content. This also serves to signal the start of a document (...)

The sops 3.7.2 (and possibly earlier too) do not preserve the --- present in YAML file when running with --in-place option.

For example, here is my test.yaml

---
apiVersion: v1
kind: Secret
type: kubernetes.io/tls
metadata:
  name: test-secret
data:
  crt: MTIzNDU2Nzg5MA==

which I encrypt with

sops --encrypt --in-place --azure-kv $(az keyvault key show --name sops-key-aks --vault-name kv-aks-dev --query key.kid --output tsv) .\test.yaml`

and here is the result:

>  head .\test.yaml
apiVersion: ENC[AES256_GCM,data:Ruc=,iv:mlHqE4A4ZSG9nOySG34gWAIN/GGqWmryS4F+/TV1VnE=,tag:HFp30nc6Eur03F7GPh/+ow==,type:str]
kind: ENC[AES256_GCM,data:xBQRIkJU,iv:EVzl7BT5nfVj7JD0eziDUQLwWwyXMyOQ6PJqntgE+O8=,tag:DnnBdBwmbOFRJdPFJy2EPw==,type:str]
type: ENC[AES256_GCM,data:LRD+jCyosIIwgFmtkgaAD2M=,iv:g35TjzFYJ4hxbW/nC2i8EbeASHkkQehd0UxXGlVkKBc=,tag:TjH30dUgCqqEOCkdR7Mnxw==,type:str]
metadata:
    name: ENC[AES256_GCM,data:fGAaeI+eKLgh4NY=,iv:RxRusjFjTnq+5Aqkoy5I3+0iqXuEeiArqGwB0gBJDqE=,tag:nF+ibSJvYYy6GJS4C59b1w==,type:str]
data:
    crt: ENC[AES256_GCM,data:P7WgLE0BIVZMBRYjsnIUZQ==,iv:TvdrJ9RLhG3UPZZlzMlOPeM6YsON+aOXfSCkmMKx1VQ=,tag:t9ovrrUsJQ+YYlHufIkkAQ==,type:str]
sops:
    kms: []

This is not a big deal but an annoyance for those using CI/CD linting their YAML-s with https://yamllint.readthedocs.io/en/latest/rules.html#module-yamllint.rules.document_start

Reproduce the issue with the provided test.yaml and the sops --encrypt --in-place command, then trace the YAML encryption and serialization path. Verify that the document-start marker (---) remains in the encrypted output while the existing data is still encrypted and valid for yamllint.