Yeah the IR SAM threat is not a new thing, 25 NATO coalition aircraft were damaged or downed by IR SAMs during the Gulf war, and that was three decades ago. The IR SAM threat has been understood since the SA-7/9K32 Strela-2. This is why IR signature reduction is so important to stealth/low observability technology.

What's new are these frankenSAM systems in Yemen and Ukraine using advanced infrared guided air to air missiles with high off boresight capability like the R-73, ASRAAM and latest AIM-9s as SAMs, and advanced ground based infrared search and track systems that can connect to more traditional SAM, which extends the range of the IR threat considerably.

An F-35 is not going to be as good as something like the F-117, B-2, B-21 or YF-23 prototype at hiding it's engine exhaust from ground based sensors, it's not even as good as the F-22 at that, nevermind those previous aircraft where the engine exhaust isn't even visible from below. Such was likely one of the compromises in the F-35s design, to allow for mass production and fulfilling all the different roles all 3 F-35 variants carry out.