Lets Encrypt fails for this regex matcher HostRegexp(`{subdomain:[[:ascii:]]+}...`)

Traefik Traefik v2
1

This otherwise valid regex matcher fails to pass traffic and obtain Lets Encrypt certificate:

- 'traefik.http.routers.dokku-http.rule=HostRegexp(`{subdomain:[[:ascii:]]+}.dokku.${SERVER_HOSTNAME}`)'

and this works fine:

- 'traefik.http.routers.dokku-http.rule=HostRegexp(`{subdomain:[a-z1-9\-]+}.dokku.${SERVER_HOSTNAME}`)'

Why is the first one failing, it's a valid regex, is there an explanation?