As far as I know, VRChat uses additional cameras to get what mirrors are reflecting.

I have found that culling by far plane is not working for those cameras.

I prepared a small world to reproduce: https://vrchat.com/home/world/wrld_6691eebf-4d71-4290-8133-1b2264c2fdb8

This world has VRCMirror at (0, 0, 0) and a sphere at (0, 0, -2000), and the far plane of the reference camera is 1000.

The sphere will not be rendered anyway because it is too far from players if it behaves as expected. However, the sphere can be seen in the mirror.

Sometimes this issue causes unexpected performance degradation when a world or an avatar uses culling to reduce draw calls.