oprypin mkdocs-material==9.6.10 navigation bars no longer support section-index behavior; instead, the update makes the theme use the default behavior even when section-index is installed.
mkdocs-material==9.6.9 is the last version of the material theme that supports section-index navigation bar behavior.
There were significant changes to the navigation template logic.
Incoming users of section-index might find this information useful.