G-String loses sustain at 12th fret--Not serious, just weird

Something I noticed the other day. My g-string, when played at the twelfth fre,t cuts out fairly suddenly after a couple of seconds while the other strings at the twelfth fret are still ringing. There's no buzz of it hitting any higher frets, and it sustains as long as the other strings when it's at any other fret (I think--Haven't tried 'em all).

This isn't really bothering me, I'm just curious as to why it's happening.