I have the exact same problem plus its getting worse.
I reported the reverse squealing problem during my 16,000 Kms service. It actually started around 12,000 Kms. Its extremely loud and is the worse when reversing at very low speeds on a cold morning with the wheel turned while backing out. Service technician said that they couldn't replicate the problem.
At 20,000 Kms, I started to have a different type of squeeling, less loud and not as high of a pitch when pressing on the brake fairly hard, when traveling at very low speeds almost coming to a stop.
I just took in my 5 for the 24,000 Kms service today and stressed that the problem is now happening in both forward and reverse. This time they easily detected the problem but said that they need Mazda Canada to decide whether to fix it under warranty or not. They will let me know their decision in a week. They said the issue is possibly due to the caliper brackets and they may need to be replaced.
I'm gonna stay firm and and insist they replace or fix the problem as I think its unacceptable. My 3 previous Mustangs lasted close to 20 years and never had this type of problem before.
I was happy with Mazda prior to this issue and now that I know others are having similar issues, I think Mazda should come clean and address the problem by fixing it. I wasn't impressed how they denied there wasn't an issue the first time I reported it.
My 24,000 Kms service indicated I have 6 mm in the 2 front brakes and 8 mm in the 2 rear brake pads. Mazda said to replace them at 2 mm. So they know the brake pads are not the problem.
I'll give a follow up in a few weeks.