root_level: 1 not honored when there's only one navigation file without a list title

I want to get a separate PDF for each top level navigation entry, set root_level: 1, but at some conditions I only get one PDF.

Some discussion and @mojavelinux's assumption about the reason of the problem: https://antora.zulipchat.com/#narrow/stream/282400-users/topic/Antora.20Assembler.20doesn't.20generate.20expected.20amount.20of.20PDFs

Steps to reproduce:

1. Clone https://gitlab.com/antora/demo/docs-site.git, do the rest of https://gitlab.com/antora/antora-assembler/-/blob/main/README.adoc, set root_level: 1. PDF generation works as expected.
2. Add new page PageC to Component B, add it to navigation. PDF generation works as expected, new file page-c.pdf is successfully created.
3. Remove Module One from navigation. Expected behavior: module-one-overview.pdf disappears, but page-c.pdf remains. Reality: both module-one-overview.pdf and page-c.pdf disappear. At the same time, PageC is still shown in the website's navbar.

I have forked Component B with the changes from (2) and (3) to make it easier to reproduce: https://gitlab.com/poglazov/demo-component-b