Quoted from the Norstar manual:
With the default Basic setting, answer keys do not ring for:
• Prime set call capture
• call forwarded calls
• Other answer key calls
• Priority calls
• Voice calls
• ringing service
• Callbacks
The Enhanced setting is the same as the Basic setting except that answer
Keys will ring for overflow call routed calls (special case of Prime set call
Capture). Answer keys will not ring for line transferred calls (although
Basic will ring).
The Extended setting allows ring for all call types except priority calls and
Voice calls.

As you can see it should not ring any extension when you press the answer DN button in any setting, so its just one of the bugs in the s/w.That’s my opinion anyway.