Persistent "Parking brake malfunction/unavailable" messages

I originally posted about this issue here, but I'm creating a new thread because the problem has evolved.

I bought a 2021 Sienna about 2 weeks ago. After a week and a half of driving, I see a "parking brake malfunctioning/unavailable" message on the dash. Then the car dies one night. I'm able to jump it with a starter pack and I take it to a dealership. They diagnose a bad 12v battery and clogged hybrid battery vents. I take it to another dealership and they diagnose a bad ECM. That dealership replaces the ECM, 12v battery, and cleans the vents. I drive the car home today about an hour and everything seems fine. A few hours later, I drive it again and within 10 minutes, I see the same "parking brake malfunctioning/unavailable" message on the dash. I park the car for an hour and drive it home—the message on the dash doesn't go away.

After parking the car at home, I read online about disabling the auto EPB. So I start the car and hold the parking lever/button down, and lo and behold, the exclamation point and the message goes away from the dash. I try pulling on the parking lever/button and it doesn't engage at first, but after a few tries, I actually get it to engage. Then I disengage it and pull and hold on the button/lever to enable the auto EPB again and the malfunction/unavailable message reappears! After all this, I'm unable to disable the auto EPB again—everytime I try, it just spits out the same malfunction/unavailable message.

After reading up on this, it seems like there are about a thousand possible causes of this problem. When I got home, I measured the voltage of the battery with a multimeter and it read at 12.87. I'll measure it again in the morning. Given all this, what's most likely to be the culprit? I'll call the dealership in the morning again, but I want to have a better sense of the issue before I bring it back to the dealership for the third time since owning it.