Traefik selecting wrong certificate

I believe I'm describing a bug.

I have a problem on selecting the certificate when both letsencrypt and static certs are present.

I have a router defined on file as this:

      entryPoints = ["web-secured"]
      middlewares = ["add-www"]
      rule = "Host(``)"
      service = "aca"
        certresolver = "leresolver"

Note: the cert is correctly obtained from letsencrypt.

I also have defined the following:

    certFile = "/etc/traefik/certs/wildcard_2021_my_domain.crt"
    keyFile = "/etc/traefik/certs/wildcard_2021_my_domain.key"

but this certificate has expired.

My idea is that the router mandates that the certificate is to be obtained from Let'sEncrypt so that that cert is to be used.

In reality traefik serves both of them in different moments (I did not understand on which base).
Moreover it servers it even though it expired...

Is this a bug or this behaviour should be expected?