Kukutanov,
There are 2 possible reasons for this.
1) The allowable compressive stress is governed by local buckling of the components of the cross section, rather than buckling or yielding of the gross section. So, changing the KY, which affects only the gross section capacity is not having any effect. Add the TRACK parameter with a value 4. Then examine the output. The allowable stress for the gross section checks as well as the cross-section-component checks will both be reported and you should be able to confirm if this is the cause.
If the above is not the reason, the following may be why.
Under the heading "AXIAL COMPRESSION - GROSS SECTION", the program will report three slenderness values :
KL/r-torsional , KL/r-Flexural-torsional , and, KL/r-Flexural
The highest among them wil also be reported as KL/r-Maximum, and, the allowable compressive stress based on checking the gross section will be based on KL/r-Maximum
It may be possible that when you set KY to 1.0, KL/r-Maximum is KL/r-Flexural, but when you set KY to 0.5, KL/r-Maximum is KL/r-torsional and these two maximas are perhaps very close resulting in very similar allowable compressive stresses. If you eliminate torsional buckling as a governing condition, KL/r-Maximum will be equal to KL/r-Flexural for KY=1.0 as well as for KY=0.5. You can do this by setting the parameter KT to a small number like 0.01. You should then see a significant increase in the allowable compressive stress when KY is 0.5.