I doubt heavily it is some issue with the car's encoding. It is working as advertised. Playback is a result if some codec in Mac not handling the stream properly.
The various stutters and compression artifacts are likely due to a a slow USB drive. I would try to find one that is proven to be faster. If it is still having compression artifacts and stutters, it could be a problem with the USB port or hub in the MCU not having the bandwidth available. It also COULD be a slow or poorly operating encoder in the computer's OS. That could improve with the next software update. But I would focus on the memory medium speed. If you can put a higher end SD or MicroSD card designed for heat and speed, (cars get hot and that is not good for some memory), and it is still having issues, it may be something with the USB on the car's computer or, the software, or the computer itself.
Since this is h.265 format, you obliviously have the newest AP hardware. So it should have more than enough power to process some relatively low resolution streams from 3 cameras.
On a side note, I use microSD cards (as stated before, hardened for hot environments), but have tried a few different USB adapters for them. I use ones that have dual SD card slots, so I can have TeslaCam on it's own SD card and .mp3s on their own card. This way, the video is not trying to write to a file system that is also being read from at the same time. It could probably handle it, but wanted to maximize per channel bandwidth for the video. The card readers I used were not all created equal. I found a cheap dual micro SD reader stick with USB3 on one end, and USB-C on the other, that allowed both cards to be addressable at the same time. Thought it would be perfect for my use case, but the cheap USB dual micro SD reader was not fast enough. TeslaCam footage had a lot of stutters and artifacts. Changed back to my previous USB 3 cabled SD/MicroSD card reader and it was fine again. Not all hardware is created equal.
I am currently on a quest for the best USB dual SD/microSD option. I think I will use a stick with low profile SD to Micro SD adapter with USB-C interface. and then just use a short USB-C to USB3 cable. This way I can quickly unplug and view files on my Android phone if I need to.
I also ordered a Raspberry Pi. Planning to try to make a wifi system that I can pull data from my home network when I get home each day without having to unplug anything. Or... Elon could release an update to the OS to allow you to access those attached USB storage slots from your wifi.