Hi glen, thanks for the info.
We use darkdb theme. I changed theme to "stock" clean (in 2.7.1+), but the problem still exists, so it is not a theme issue.
Editing teacher can hide a course section, but after refreshing the course view, it cannot be made visible again.
I've made a brand new course with four sections. In the first section I added one label with typed "This is Test" sentence (so as to avoid copying from MS Word, for instance). The problem still exists