![Image](https://forums.factorio.com/images/ext/aaabab90c075f65a8c991e58aa8ef35d.png)
As you can see, the setup is symmetric from the outside. I started both chests with 300 copper plates, but the left chest (red wire) only filled up the right side of the belt and had to stop, while the right chest (green wire) is constantly putting onto the belt because both inserters taking from the underground belt are grabbing the plates on the left side.
The same setup was working fine in 0.11.22, with inserters always pulling from the close side if possible, so this seems to be a bug to me.