Since I was shooting at a target overlapping another I didn't realize it does automatically cycle targets.  But It sure enough did veer off after a Su-27.  I know because it was a friendly I was on comms with and was looking at the map.  I told him to turn off his radar because he was oblivious.  It would have come close to hitting him if I didn't tell him to turn off his radar.  The missile died not far from him but failed to track after going nose cold.  I also saw that B-52 / HARM incident and it was scary similar.