It has a tendency to change but it would *change* whenever the away status change, yet this one is available via the prop.
Possibly the away status is available for mIRC without asking it to the server, which may not be true for the away message, but that could still be wanted by user, for example ircv3 might improve this in the future. $ial().awaymsg can always be a thing, even if it wouldn't return something until you whois the user (at least for now)
Looking for a good help channel about mIRC? Check #mircscripting @ irc.swiftirc.net