Call it a hunch, but if the flares spew dirty and irregular, maybe there's an awareness that this sloppy spread of IR readings has benefits.
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.
Given the age of the software in these missiles, the issue is likely relatively weak and brittle classifiers being used to separate noise from signal. It is not based on any kind of modern deep learning techniques.
This reminds me of a story where the USA got its hands on a new MiG and when they opened it up found that it used some kind of antiquated technology. Some time later the USA found out that this was intentional because the older tech could withstand nuclear radiation whereas the new tech would instantly fail.
Fun history, the Russians demanded it back off the US/Japanese so the Japanese government let the US take it apart, photo and test the parts (as quickly as possible) then shipped it back in crates and billed the Russians for the shipping. Which iirc they never paid.
"Unable to afford the thermal-resistance provided by a titanium frame, the MiG-25’s designers used vacuum tubes over transistor-based modern electronics — which made the planes resistant to high-temperatures and cheaper to maintain."[0]
[0] http://warisboring.com/one-soviet-defector-shoved-japan-into...
USSR could have replied by sending an invoice for the rental of the plane, reassembly of the components, and the intellectual property licensing for all of the technical data obtained. All of that would no doubt ought weigh the cost of crating and shipping the plane by orders of magnitude. Not that either side would ever have considered paying, of course.
In reality, some entirely secret concession could have been made that we might never find out about unless it was documented and is some day declassified.
There's a similar story about the "sloppy" riveting work on the Mig-25. Turned out to have significant beneficial effect in the airflow.
That was brought home to me in a documentary I watched about the Mig-29, the US/Nato avoided FOD (Foreign Object Damage) by scrupulously walking the runway/apron every day and cleaning up.
The USSR just put air intakes on the top for taxiing so crap didn't get sucked into the main inlets.
It was such a simple design and yet the advantages in a war situation operating from crappy debris strewn airfields where obvious, they had Mig-29's taxing over rough dirt berms and all sorts of crazy things (at least from a western viewpoint).