Lets take the example of Python 5,ASRAAM. They have "imaging" Infra red seeker.They can now see an aircraft and match it with its records in order to differentiate it from a flare.
Other WVR missiles have Infra red counter counter measures as well.
I would like to see a credible technical source for that claim. I am willing to bet dollars to doughnuts that you have the incorrect context for 'imaging' here.
BVR weapons like AMRAAM and others have "Home on Jamming" capability.
Many good air forces now have better missiles in their inventory.
Flares are not 'jamming' in the strict sense. It is seduction.
The way a flare works is:
1- Present to the missile a greater than IR source compared to the aircraft that launched it.
2- Saturate the missile's field of view (FoV), which is limited due to space constraints.
Other factors to consider, regarding your claim, is if the IR sensor is of the scanning type, whose weaknesses are complexity, which would require a larger missile body, and low frame rate. If the IR sensor is of the staring type, then its weaknesses are poor spatial resolution, require multiple arrays to have effective coverage in the event of a maneuvering target, and because of this, the staring type have a high processing load. Either types affect fusing distance, which is another can of worms.
Early IR sensor technology had difficulties distinguishing multiple IR sources on a body. Today's sensors are able to discern these multiple sources and so the intention is to focus in the largest IR source, which would be the engines. But this would make the missile useless in anything situation other than a chase as in a frontal view, the target aircraft has its leading edges as IR sources and they are nowhere distinct as a jet engine.
So if the missile is designed to focus in on the largest IR source it could find, odds are better than average that it will be seduced by a flare even though there might be spatial separation between aircraft and flare. If the missile is designed with target adaptive guidance (TAG) algorithms that uses multiple IR sources on a body, then it will be vulnerable to saturation tactic, which would involve multiple flare launches.
Remember, this is not active like radar but passive, meaning the missile is totally reliant upon the target itself for target information based upon an emissivity type that is target control, not missile control. Currently we do not 'beam' an IR emission towards the target like we do with a radar beam, do we?
What type of 'lock-on' are we talking about? IR emission intensity is inversely proportional to distance and that inevitably involve signal-to-noise ratio. With lock-on-
BEFORE-launch missiles, human intelligence is required, meaning the pilot must remained focused on his target until the missile signaled him that it has acquired an IR source, which lead us back to sensor type, scanning or staring, and their associated weaknesses. With lock-on-
AFTER-launch missiles, low SNR is an issue and the system is most vulnerable to saturation tactic. The American anti-tank Maverick missile is lock-on-after-launch (LOAL) but it is against ground targets, whose domain is two-dimensional, hence limited in maneuverability.
Not as easy as you think for a passive sensor system to avoid seduction.