TNX for reporting back the solution
!
Indeed that selector is in https://github.com/moodle/moodle/blob/06e3b6d8bab42b8e56d169d006f31f4a15684830/theme/boost/scss/bootstrap/_print.scss#L17.
You could file an issue in the Tracker, quoting your entire post there.
TIA,
Matteo