Turns out this is just how Traefik handles routing when the regex can't match anything.
Since I was redirecting every http request to the server to my regex replacement, but only matching with regex those that specifically include the domain name, any requests directly to the IP address would hit the same redirect but not match the regex.
# matches only requests with the domain name
- "traefik.http.middlewares.test.redirectregex.regex=https?:\\/\\/(.*\\.)?starthubs(?:\\.co)?\\.uk(.*)?"
- "traefik.http.middlewares.test.redirectregex.replacement=https://$${1}starthubs.uk$${2}"
- "traefik.http.middlewares.test.redirectregex.permanent=true"
# sends any http request to the middleware
- "traefik.http.routers.http-catchall.rule=hostregexp(`{host:.+}`)"
- "traefik.http.routers.http-catchall.entrypoints=web"
- "traefik.http.routers.http-catchall.middlewares=test"
I fixed it by changing the router rule to only redirect if the domain name is present.
- "traefik.http.routers.http-catchall.rule=hostregexp(`{host:(.+\\.)?starthubs(\\.co)?\\.uk.*}`)"