"generate ToC from all headings" doesn't save newly created entries
 

In edit toc, I try to use generate toc from all headings to automatically create toc. the creation process has no sign of error. But when I click Ok, none is saved. When I open the viewer or edit toc again, all previously generated toc are gone.

here's a video clip showing my operation. is there anything I did wrong?

https://youtu.be/fE3lxtOG9Rg

edit:
Actually, in the version 5.19, all edition of TOC doesn't save. When I roll back to 5.18, it works.