CSS Editor not saving selector as expected

I’ve been using CSS Editor Plugin for Omeka:
https://omeka.org/classic/plugins/CSSEditor/

A CSS selector that i put as input is not been saved as expected.

The rule I’m trying to save is:

#search-container button {
background:#731022
}

When saved, it becomes:

#search-container {
background:#731022
}

Removing the button selector.

Maybe it’s something related to this topic:

Or this issue on github:

Saving directly to the database works, but the next CSS update will remove the selector again.

Any hint to solve the problem?

Should be reported as an issue to github?