The type specified in "Modeling Preferences" > "Body Type" is written as "Specifies the default type of body created with certain commands".
And it is clearly stated that the "Sew" command is also the target of that certain command, but the specified value is not reflected in the "Sew" command dialog.
https://docs.sw.siemens.com/en-US/doc/209349590/PL20200605195244930.modeling/preferences_prefs_modl_gen
Reproduced Steps:
1. File > Preferences > Modeling > General > General Construction > Body Type=Sheet > OK
2. Surface tab > Combine group > Sew
3. Conform Body Type in Settings of the dialog
This behavior is due to NX problem and has been fixed in NX 2306.8700 and NX 2312.
Workaround : Change this on the dialog every time while performing Sew command.
Notes**Please refer to PL8783440 for Japanese article.