There have been hundreds of posts on this forum and other EV and car forums by folks with weird issues that were caused by weak or old batteries. Even brand new batteries from the dealer or off the shelf at auto parts stores--they are sulfated from sitting on the shelf for an extended time during these pandemic times. That is why i suggest you do a 15 second load test at 150 Amps to check and verify the condition of the "new" battery. Even better is also do a capacity test.
You have made 2 assumptions: that the dealer would check the new battery before installation, and that it is "good" just because the OCV is 12.7V. i have bought 4 new 12V batteries over the past year and none of them were fully charged even though they read 12.7V. They all needed to be charged up to 14.4 to desulfate the plates and reach a full charge level, after which the new voltage was 12.8 to 13.2V. 12.7 is not a full voltage charge of a new battery.
Once the possiblility of a battery issue is completely ruled out by verified performance, then you can address the CAN buss issue, if it still exists.
i would consider exercising the brake system by driving fore and aft in the driveway, or slowly around the block on a side street. Accelerate up to 15-20mph, then hit the brakes, then let off; drive around and do about 50 cycles. Listen for the motor actuator squealing or any other weird noises. The motor advances the ball screw under braking, then retracts when you let off. If the car has set for awhile, then the brake motor actuator may have some surface rust, debris or corrosion that needs a good work over to get back to shape. Look at all the bearings, springs and sliding surfaces that could contribute friction to the actuator--it's a complicated piece for sure. Good luck on getting it sorted out.