When I add Re: or Fwd: to the beginning of a subject line the email proof and the deployed version removes it. We haven't used a subject line like this for maybe 6 weeks or more so I'm not sure when this began happening

Just wondering if anyone else is having trouble with "re" or "fwd" in the subject lines recently? We've used them in the past without issue. Typically we use them only in the case that there was an error within the email