1.Start Origin, new 2 workbook, fill data
2.use one of them to create graph
3.open PD, go to label tab, select enable, select show at specified indices only
4.click dropdown under this checkbox
--> columns in dropdown are correctly
5.select File: recent project: select any project
6.double click a graph in new graph,
7.repeat steps 3~4
==> columns in dropdown is same as last project, not current project

Fixed in Origin2020b.