Usually you would want the flare to mimic the spectrum of your own engines. I don't know if by irregular they meant in spectrum or illumination level. There is

Usually you would want the flare to mimic the spectrum of your own engines. I don't know if by irregular they meant in spectrum or illumination level. There is not a lot of short term variation in a jet exhaust due to thermal mass and the need for thrust to keep maneuvering, but the aspect changes. So sensors are not assuming a straight tail chase. I consider it much more likely that the Russians at some point obtained some AIM-9X seekers and tweaked their flares and/or engines to make it less effective. The is also the concept of the 'war load' or 'war paint'. In normal operation planes will have their radar cross section increased and corner reflectors added, engine burn is tweaked, etc, anything that could be leaked or sensed remotely (when flying in international airspace, or from space, or covertly through radar backscatter). When you think things will get serious you pull out those chunks of superfluous steel, tweak the engine exhaust nozzle, load different firmware onto the missiles and the EW system. If actual combat occurs there may be different war-only RADAR wave forms. Probably none of those are available to Syria (and the US has been building RADAR signatures of them for many months), except maybe an upgraded flare dispenser.