This affects course formats for Moodle 3.11 (e.g. Grid format) when running in Moodle 4.0 development.

Steps:
1/ Have current Moodle 4.0 development.
2/ Install course format for Moodle 3.11 (e.g. Grid format)
3/ Create course in the format, with at least one section.
4/ In section 1, add an activity, e.g. a Page activity.
5/ Hide section 1.
6/ Note the activity under section 1 appears shown, when it should appear hidden.
7/ Show section 1.
8/ Note the activity under section 1 appears hidden, which is should appear shown.

I think this may have something to do with course/format/classes/base.php function section_action(), although I'm not sure.  I think the same function is used for Moodle 4.0 course formats without issue.