Thought this might have been a one time thing when I first observed it, but I've now had it happen 3 times, and observed it happen to another player.
Once AMS starts firing at missiles that are within range, but track out of LoS, it seems that the system keeps firing at missiles that it thinks are still there. Seems to have only started happening post patch now that the AMS range has been increased. This has been hard to deliberately replicate as it relies on other competitive players trying to kill me. Also, it seems to only affect my AMS when it is spotting for a friend, as I never get the incoming missile message, but hear the AMS chattering, and then not stop.
Twice had it occur on Canyon Network and observed a third occurrence there, where a friendly ShadowHawk also fired till it ran dry. Much rage ensued. Also experienced it on HPG myself.
Going to guess here that it has something to do with the sharp terrain geometry.
All instances were in my Firestarter with twin AMS.
Let me tell you, watching 2000 rounds rapidly spin down to 0 near the start of the match when you're relied upon for shielding the line mechs from LRMs is not a fun experience
