Blind zone indicatorTraveling earlier this week (1265 miles), the Blind Zone indicators were inconsistant when passing other vehicles..
Driver side worked okay.
On the passenger side most cars, vans & pickups invoked the indicators (outside mirror & heads up) but not always for these vehicles. Semis were a different matter, most of the time I did not get a indicator.

Prior to this trip, I had never notice this condition. Any assistance would be helpful.

2021 Mazda CX-30

 

I noticed similar behavior on my CX-5, and had actually scheduled an appointment to have the dealer look at it. However, after driving it on the freeway a few more days, I canceled my appointment. I think I figured out the way their BSM system works - the software seems to be smart enough to detect whether you're passing a car quickly or slowly, and doesn't give you an indication on a "fast pass". I thought my passenger side sensor may have been defective (or a loose wire), but after trying different passing scenarios, the indication does seem to show up (consistently) whenever a vehicle is camped out in my blind spot, or if I'm passing it slowly. Maybe this is Mazda's response to complaints of overly-sensitive blind spot warnings on earlier models?

 

I had a Mazda look at this... they could not duplicate the issue but did tweek the sensitivity. Every since it appears to work correctly.
I had never seen the issue in the prior 13 months of ownership... only occurred on my long trip in mid-Oc tober.
Your theory might have some validity as on our lon trip many times we were traveling at 75-80 mph. But since arrival we had notice the issue in city driving, until the dealer made the adjustments.