Hi,
FWIW, I do not think this is related to any changes on the car stereo. Effectively all this started after an iOS update (13.5 if I recall). Most people have the same stereo in the same car that has undergone absolutely no changes in years. The sheer volume of different vehicles and stereos all suddenly experiencing the same issue points squarly at Apple. Further, I suspect that this is something being manipulated on the back end of Siri's performance by Apple, simply because there are so many people that experience cycles of normal and abnormal Siri behavior between iOS updates.
For example, I have a Ford Kuga ("Escape" in other markets) - it has had precisely 2 software updates since new in 2015, both years ago. I have used numerous iPhones with the car stereo, all of which worked perfectly - until the iOS 13.5 update on my 11 Pro Max.
We all know what car manufacturers are like (Tesla being the rare exception) - effectively the equipment you get in the car stays pretty much the same for the life of the car. Certainly, I would find it unbelievably coincidental that a multitude of different cars from different years and manufacturers, all containing stereos with similar diversity, all start misbehaving with Siri at the same time and in the same way.
Statistically, there are going to be some folks in this thread that have issues that may not be related to the Siri / iOS update, but I think the vast majority simply noticed a problem start at a point in time with the only common theme being an iOS update.
Cheers,
Mark.