If you look in the GTAC solutions there is information.
As of NX11.0.2 MP5 and NX12.0.1 the actual usage of upper/lower case in material attribute vs. material assignment is also considered, i.e. "ALUMINUM" and "Aluminum" are no longer considdered to be the same.
In your specific case this renders in the material attribute being considered to be incorrect and is changed to the assigned material when the part file is opened.
As a workaround the Information window with its message can be supressed by setting the variable UGII_SHOW_MISMATCHED_MATERIAL_ATTRIBUTES_IN_INFO_WINDOW=1
Setting the variable however have its limitations as it also mask away the message in cases where the "Material" attribute have been assigned a value manually, without assigning a material to the solid body.
When the variable is set the message will still occur in the NX log file.
If you open up this Reuse Part in an earlier version of NX (say NX10) you will see that the value assigned to the Material part attribute is STEEL. Now that attribute values are case sensitive, NX will compare the value assigned to the Material part attribute with the materials in the Materials Library. Since the Materials Library only has "Steel", not "STEEL", NX reports the inconsistency in an information window. In newer versions of NX, the Material part attribute's value reads as "Steel" when the reuse part is opened - which is why the inconsistency is not immediately apparent.
As @sdeters explains, you can mask the issue by setting the undocumented environment variable. In my opinion it's better to correct the issue (though I realize this could be a daunting task if you have to correct many std parts), or just close the information window when the inconsistency is flagged.
It's my understanding that a similar message is raised if a part is not assigned a material part attribute, or is assigned the material part attribute but the value is empty. ER 9206007 was submitted asking that a more informative message be written to the info window in these cases.
@BenBroad while your at it, we have noticed an increase, or at least what appears to be an increase, in windows popping up, asking which material we want to apply to a part, when opening assemblies. The additional material is due to wave links, and it's very time consuming to have to click "cancel" to prevent an accidental assignment of an incorrect material. Currently I have two ER's on this 9169664 and 9107468, and I'm curious if this is related to the NX NX11.0.2 MP5 change, or just coincidence.
FYI, I uploaded a video, which shows the amount of selection boxes we have to go thru.