Well, you're technically not disallowed from still doing traffic over HTTP if you've implemented HSTS. It's just an indication to the client that they should always contact the site over HTTPS.
The only thing I can imagine is that the Strict-Transport-Security header wasn't cached yet, and after forcibly connecting won't get applied because the connection is already insecure.
@NautArch Don't think so. Strange thing is, the site is sending a Strict-Transport-Security header, so Firefox obeys it and doesn't allow one to go further... Chrome on the other hand seems to allow me to ignore it, which is kind of scary.