Highlighted Display of Shaded Features Looks Incorrect and Unexpected. The pre-highlighted feature has more shaded faces than expected.
Solution What might be highlighted is the original feature that existed prior to the current feature when pre-highlighting features before selecting them by having the option "Highlight Selection on Rollover" which is checked on by default.
A quick method to determine if this is the case is to uncheck the file specific option 'File -> Preferences -> Selection -> Highlight Original'.
From the NX documentation: Highlights the original object from which the existing object has resulted, when you move the cursor over the object.
This can also be disabled for a user by selecting 'File -> Utilities -> Customer Defaults -> Gateway -> General -> Selection (tab) -> (uncheck) Highlight Original'.
Hardware/Software Configuration Platform: INTL64
OS Version: 10_1809