If you search the LO Writer documentation for "hidden" or for "hiding" or "hiding text" - you won't find a result about marking characters as hidden using the Character properties dialog. In fact, you will find information about hiding text using field codes - but not a word about the properties dialog. See, for example: help/en-US/text/swriter/guide/hidden_text.html?&DbPAR=WRITER&System=UNIX
The right Help page is https://help.libreoffice.org/master/en-US/text/shared/01/05020200.html?DbPAR=WRITER#bm_id3153514 I'm adding some extra bookmarks.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/5f61c7de68a7ca48e2aea2827fcb8c081100d581 tdf#154121 Add hidden characters to index