I just created work parts for Bartók's Rhapsody no. 2 for violin and piano, Sz. 89 (a412df25-c5a2-4d1c-a9b8-bac1876fd360) from this work's edit page.

I first create both parts as new "parts" relationships.
When I check "These relationships have a specific ordering", the first one disappears from the page, only the second one remains (with the correct "2." numbering)

The data is correctly saved when submitting the edit