Traefik HostRegexp does not match [closed]

I want to use HostRegex to forward all matching subdomains to a specific location. This is my dynamic config

http:
  routers:
    my-router:
      entryPoints:
      - web
      - webSecure
      rule: "HostRegexp(`{subdomain:[a-z]+}.test.example.com`)"
      service: my-service
      tls:
        certResolver: zerossl
        domains:
        - main: "test.example.com"
          sans:
          - "*.test.example.com"
  services:
    my-service:
      loadBalancer:
        passHostHeader: true
        servers:
        - url: https://srv-02.int.example.com:8443

But it does not match https://automation.test.example.com.

I had a look into the Traefik sources and put a test next to the existing ones and there it seems to be OK.

If I change my rule to HostRegexp(`automation.test.example.com`) then it does match.

I am using Traefik 3.6.0.

Any ideas?