Voxel units cannot be repaletted by using Palette

That's probably because voxels in general are immune to shp palette changes.

I amn't sure about it. Voxel building turrets are repaletted, as my report stated and shown on a screenshot.

Maybe the exception enforces the rule, as we, Hungarians say.

Even though some Building Turrets are vxl, I believe game reads them as Animations since they are listed in [Animations] and use the TurretAnim= tag. That's probably why your able to use custom palettes on those I would assume.

Maybe make the game think all vxl are animations it might work? I mean they are partially animations based on sprite sheets anyway right? I don't know, I'm probably reaching for the stars on this one, huh? lol

Since a new bug about this has been reported, i think it's necessary to mention that voxel AircraftTypes (they can be only voxels) DO support custom palettes. It's only voxel vehicles that don't, for some reason.
Perhaps related to voxel image change behavior? (hardcoded APC.vxl and APCW.vxl, "loaded" V3 launcher and "unloaded" V3)

Keep in mind that voxel aircraft are pretty much stricter as well - the missing-HVA crash comes into my mind, which suggests me that vehicle rendering was probably the heavy testbed for voxels and thereby were overklooked for updates when the rest got - aircrafts probably got the buildings' code there.