I tried to setup atlas texture randomization in decals, but i found that UVW randomizer gives unexpected results if decal mode is used in conjunction with mesh element mode. The setup works with either of those modes alone.
Select any UVW randomizer map in the attached scene and turn on/off mesh element mode. Watch how result is breaking when both decal and mesh element modes are on. Don't mind the missing bolts - that is by design.
P.S. i'm not sure, but i think i did similar setup in the past and there were no issues with it, so it could be that the bug is pretty recent.