Export Parts creates one part that includes all parts. This happens in the individual part file and also in the "score and parts" file.

1) Create a score with several instruments. In this case, KB, G1 (guitar 1) and G2 (guitar 2).
2) Save as PartsTest.
3) Execute File /Export. This creates a correct PartsTestExport.pdf file, showing the 3 instruments.
4) Execute File / Export Parts. This creates 4 files
a) PartsTest Export Parts-Score_and_Parts.pdf
b) PartsTest Export Parts-G1.pdf
c) PartsTest Export Parts-KB.pdf
d) PartsTest Export Parts-Guitar_2.pdf
5) Files created in c) and d) are CORRECT, showing ony one instrument each.
6) File in a) is INCORRECT. It shows first the combined score, but in the section for G1 it shows the 3 instruments.
7) File in b) is INCORRECT, showing the 3 instruments.