Sub-libraries disappearing?

I created a bunch of sub-libraries and added books to them (in one case, using the bulk add feature).
When I went back to the library, the sub-libraries have disappeared from view, and the books in sub-libraries are not displayed in the library.
I can still see them when browsing storage, and can still find them via search.
"Rebuild library" doesn't help.

Has anyone else encountered this?

Boox Note 10.3 build 2018-09-05