Instead of directly opening the step file, create a new part file (any name you like) and use "file import" to import the step file to your new file.
There are no customer defaults to prevent this, nor does the stepXXXug.def file contain a setting to control the suffix. An existing enhancement requested has already been submitted:
ER 6613286 - Option to control suffix to filename of NX prt file result from opening stp file
You can contact your local GTAC office and ask that they add you to the ER so that you are notified when the functionality has been implemented. While looking at previous issues, I noticed one explanation (for JTs) which suggested that the suffix helps to prevent overwriting the original part file . It is common for users to export to another format and then open that file to check that it has the required geometry. In doing so, the corresponding prt file is created, which if it had the same name as the original prt file it could overwrite the original part.
Use @cowski1's solution for a workaround.
It's an extra step, but when you file ->open a STP assembly you can then Clone the assembly and give it whatever prefix and/or suffix you want.
Elmhirst Industries, Specializing in Prototype sheet metal stampings and assemblies
Production: NX 12.0.1 / Autoform r7 /WorkNC 2018 r2
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@R_Newcomb's post reminded me of a similar thread where someone wanted to remove the file suffix when cloning. This is possible, but requires a simple edit of the clone log file.