The problem is not modern missiles, they have an IRCCM sensor that knows how to differentiate or try to mitigate aircraft flares, this is not implemented in DCS, but they try to simulate it by increasing the resistance of these missiles flares, which is not correct and far from being something close to reality, the real problem happens with older missiles, which do not have these same systems, these missiles for the most part just follow the target with greater heat, they cannot distinguish between a flare and an aircraft, making them easily fooled with one or two flares.
An interesting point that the other guy mentioned was the case of the aim120, where it basically had no system to avoid a notch maneuver, they implemented an irccm system that temporarily suspends its seeker when a chaff is detected, when it is suspended it tries to maintain the course and route of the enemy aircraft that is being attacked, turning on the missile seeker after a while to try to reengage the target, the aim9m and modern variants basically have this same irccm system (but for flares), making them much more deadly in combat.
If you want an example of this system working, try googling war thunder irccm missiles and strategies to dodging them, the modelled it really well