Repeated rusting rotors--rear brakes not engaging for weeks (months?)

I have a recurring problem on my 2016: rear brake rotors rusting to the point of becoming pitted and destroying pads.

As I understand, it's normal for rotors to get a bit of surface rust when wet, which gets scraped off the next time the mechanical brakes engage. Theoretically this should happen every time the car is brought to a stop (i.e. mechanical taking over from regen at very low speeds). On my Prius, which is driven daily by my partner, the rear rotor rust has progressed to deep pitting on two or three sets of rotors, which then destroy the pads when the brakes are used.

I believe the problem started happening toward the end of the of the original brakes' life. We replaced all rotors and pads with aftermarket NAPA parts in January 2021. Within months, those rear rotors started to rust. If I braked hard, or put the car in neutral, I could make them engage and they'd make a rough scraping noise. The pads quickly wore out from the roughness.

I figured I may have messed up the first replacement somehow. I replaced the rear rotors and pads a second time in late 2021, being sure to follow the techinfo instructions to the letter. This time, I kept an eye on the rotors for a few weeks after the job and they were staying nice and shiny. I was satisfied they were working as they should and stopped checking. Months later, they were pitted with rust again, suggesting that at some point they stopped being scraped clean again. They recently started working in normal driving: my partner noted an increase in noise while braking and the pads quickly wore down to the backings. I can hear them scrape during slow parking lot manoeuvres so I think they're currently working as they should, but if they had been working all along they wouldn't have rusted and ground the linings down to the backing plate.

In short, it seems like some sort of weeks- or months-long phenomenon is affecting whether the rear brakes actually engage in normal driving. I'm currently debating whether to spend thousands of dollars on a dealership brake job and ask them to try to diagnose and fix it (which could backfire if they don't succeed and the new rotors rust again), or seeking out aftermarket rotors that are rust-resistant so that it doesn't matter if the rear brakes are coming in to play or not.

Has anyone else had this issue? Any idea what could be causing it?