It would be nice to find a written explanation of what you guys have observed. Without that we're left to speculate. so...............

If the airplane is flying at a high angle of attack, it's probable that the canard would be stalled out if it was locked into that AOA as well. Wings usually have camber altering surfaces like flaps and slats that permit them to function usefully at higher AOA. Canards and stabilators usually don't have those devices and have non-changing profiles. Their profiles (airfoils) are often symmetrical due to their functions, and symmetrical profiles have even less range of AOA between stall angles. So it's a given that the canards need to be kept "flatter" to the airflow to keep them producing lift efficiently. They're also operating in lots less favorable RE which would add to their problems.

It would lead that if they were to continue to contribute efficient pitch control, that they'd be operated in an efficient range of AOAs. And that'd mean that their control system would work them the way we see them being worked. And with computer aided control systems, that's not too hard to do.

But that's just a guess. The "true" answer will need to come from someone with inside information on those fighters.

So basically, just what perttime said...............