If I apply a material to a null object with children, it works.
If I apply a second material to the null, it still works.
If that second material has an opacity channel enabled, the mapping of the second material breaks and it appears in the wrong location.
Example file and render attached. The top row of keys is using a null. The bottom row of keys uses a c4d connect object to merge all child objects together, effectively treating them as a single mesh.
R20, Octane 2020.2.5 R3, geforce 3090
Affects multiple different hardware configs