2021 Prius - brakes get heavy when stopped 2+ minutes?

I'm 2 weeks into owning a used 2021 Prius LE, with 48K miles. Previously, I drove a 2010 Prius V for 13 years.

On the 2021, I have noticed when I'm at a long stoplight, say 2+ minutes with my foot on the brake, the brake gets heavy, it feels like it is pushing back at me.

I took it back to the dealership (this is a certified pre-owned Prius), they did brake diagnostics and could find nothing wrong. Has anybody else experienced this? Not sure what to do, and I can't imagine driving the car in its current condition.

 

In case anybody else has this issue in the future, I want to share the outcome. I brought the Prius to a second dealership, and they kindly had a mechanic come sit in the car with me so that he could try the brakes for himself. He noticed the issue immediately. On top of the brakes getting heavy, when you continued to apply them, the brake pedal would pop out, and he also observed this and said it was not at all normal, and nothing he had ever observed before. Thankfully, he was excited to have a new and interesting issue to work on.

Mechanic tried for better part of a day to identify the solution on his own, but could find nothing so he contacted Toyota central engineering, who searched for similar issues and recommended they replace a brake booster component. Took 5 days for the part to arrive, and then another day for the dealer to locate a specialized tool to install it. All in, it took 8 days for me to get my car back. It was all thankfully covered by the Toyota Certified Pre-Owned Vehicle Warranty. What a hassle to have to go through so much to get it fixed, I hope I will be in the clear now as I really like this vehicle other than this issue.

As a side note, they gave me a brand new Camry as a loaner during the servicing, and driving that sure made me appreciate how lovely it is to drive a Prius.