Details
The body transparency setting while opening a component in a new window is reflecting on the assembly, but once the body transparency is changed on the assembly, the transparency setting in the single part will not be reflected on the assembly.
Solution
This behavior is as designed.
In this process, the display properties of an object in child part file get transferred to the object in assembly. As long as you do not make any changes to display properties of the object in assembly, they are totally controlled by the properties in child part file.
The moment when you select the object in the assembly, changed its translucency, you made a change to the "object in the context of the assembly". From this point onwards, it is the responsibility of the assembly to remember the display properties of the object.
This is why, though you change the translucency of the object in child part file, you have not made any change to the translucency of the object in assembly's context and hence it does not reflect in assembly.
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: 1064
Product: NX
Application: ASSEMBLIES
Function: ASSEMBLY_MODEL
Ref: 002-8574844