I'm not sure who did that, but it looks like your buffer was screwed in too far, and someone compensated for it by "v-notching" or cutting a "U groove" to clear the detent pin. It also appears the other side of the "v-notch" or "U groove" has sheared off, leaving you with one side left. It should probably still hold, but the tube should have only been threaded in enough to overlap (at all) the detent's shoulder (some barely kiss the pin, others don't. Not important. So long as the tube overlaps the shoulder). "V-notching" or "U grooving" could have weakened the original tube's lip (thus why one side is apparently missing).
My opinion?... tubes are cheap. Just put a new one on. Although that set up could still work fine, I'd not be happy with that. YMMV.
BTW: some people get obsessive about detent pin coverage. You'll see this when the tube extends INTO (past) the upper ring. That should NEVER be (usually it's a tad behind the ring).