When I was creating the “Unicode Character Code Chart: Arrows (2190–21FF)” page, I found that WordPress had replaced some of the characters in the code chart with some kind of images.
I looked the page’s source code and saw the HTML that I was expecting to see. In the glyph column for the UP DOWN ARROW (↕) code point, the characters was displayed as ↕. I tried retyping a couple of the problem glyphs to see if I had inadvertently added some hidden characters. No luck.
After a little bit of searching on Google I found that WordPress has a new “feature” that automatically replaces some characters with emojis.
After a quick search I found the “Disable Emojis” plugin. The plugin’s description says, “This plugin disables the new emoji functionality in WordPress 4.2.” That sounded like just what I was looking for, so I installed it, refreshed the page, and it fixed the problem.