The Osprey book I have on US ETO soldiers indicates in its illustrations and photographs that US soldiers had rank insignia and divisional emblems on their field uniforms, but for the Korean War, US soldiers did not have either one on their field uniforms. Was this an official policy change, or what? This reminds me of how Bill Maudlin in Up Front complained about his having to wear his rank chevrons when it made him a target for enemy snipers.

In the ETO, unit insignia were often ordered removed before an operation as a security measure (they didn't want to make it too easy for German intelligence to figure out the US Order of Battle). They would be ordered sewn on when the troops returned to the rear. Rank insignia was supposed to be worn, but was sometimes not, depending on how worried a unit was about snipers, who picked off officers and non-coms.
Korea was a war of infiltration for much of the first year, and snipers were a huge threat, so it is not surprising if rank insignia was scarce, though I know of no official orders on the subject. During Vietnam, small metal insignias that could be clipped to a collar tab started to appear instead of sleeve chevrons, for much the same reason.