When did the text editor (no difference whether it’s a classic text element as in the linked video or a new text element) start changing or removing special characters? I expect anything I write to stay exactly the way I write/paste it into the text editor and also to be displayed exactly like that, i.e. diplaying the html code in the editor, not the “result” it should produce on the front page. However I noticed that after saving (or when editing an older page and going from Rich text view to HTML) some special characters are replaced like in this case a non-breaking dash. Since it doesn’t show up at all in newly edited/created text elements I tried to edit an older page and as you can clearly see in the video it first shows up correctly but when I go into editing mode and switch from rich text to HTML view it either replaces the normal non-breaking dash with a normal one or at least displays it as a normal dash instead of showing the HTML code as before (in this case ‑) – I’ve noticed the same thing happening with other things like “­” or “ ” in the HTML view of the text editor as well as multiple line breaks in the rich text editor recently.
Please see for yourself here: https://www.dropbox.com/s/bofcb13bh96ff7o/special-characters-protheme.mov?dl=0
(WP 5.0.1; Pro 2.4.2)