why do html codes show when someone else is looking at a page, but not on the editor's (even after its saved) version.

If they're trying to edit the pages, they might be coming from a browser (Safari) where the only editor we currently have available is the Classic editor, and they would see all the wiki markup that makes up a page, whereas all you see is pretty pretty formatting. If they're using a point-and-click editor, they may have clicked the "Source" button which shows the HTML markup, and if to make that go away, they just have to click it again.

If they're just looking at the wiki pages themselves, though, you might want to have someone who knows HTML go through your source code and see if somehow some extra tags got typed in, and then figure out how to delete those without messing up the formatting you have.