I've noticed this as well. Tried in Totara 15.17 (Build: 20230322.00) and was able to replicate this. A quick list comment appears and quickly disappears. Happens in Edge and Chrome. Looks like a Totara bug.
Totara Learn Open Discussions
Unable to add comment from quicklist while editing a PDF in Totara v16
Re: Unable to add comment from quicklist while editing a PDF in Totara v16