that is true for most IR missiles, 9M is different. it does not track hottest, it tracks only jet BUT if it detects flares it will stop tracking for a short time and then start tracking again to avoid locking the flare.
Radar with IR missiles is ONLY before launch, after launch the missile will act like IR missile, the 9M specifically has IRCCM in the form of short periods of non tracking if a flare is detected.
Only if it successfully locks onto the jets heat signature before.
In which case, if it encounters multiple heat signatures, it will suspend active IR tracking and resort to IOG tracking for a short period of time until the heat signature exits the seekers FOV.