As you said, it was working - what is going through my mind is "What else has changed?". e.g., did you recently update AIX? Have you changed your default shell (and there is no "hash-bang aka #!...
In the environment where:
mail -s "three word subject" user@company.com
delivers an email with the subject three, what output do you get from the command:
type mail
and from the command:
printf...