Something somewhere is running an htmlspecialchars() or equivalent on whatever you input, probably as an attempt at "sanitizing" the text entered in titles/posts/comments. You know, to keep me from just inserting a javascript tag with src='http://pwned.ru/fu.js' into a comment and have it to something naughty to anyone who loads the page.
I'm certain these are being stored in the database as an & amp;, but they're not being decoded back into an ampersand character upon display.
Yes, it's annoying. However, there is a fullwidth ampersand available, as well as lots of other curly characters. I tried adding combining backslashes to those that miss that final stroke of ‘&’ but I gave up as they don't align consistently across platforms.
Yes. The first one is a standard diacritical circle as seen in Swedish, the other two are a combining “o” above. The middle one is a standard alpha while the others are math script bold alphas. This is an attempt to create an ersatz “&” without the character 0x26 that appears as & in post titles and monospace text. The fullwidth ampersand is obviously the closest but it appears with space around it so “D&D” looks more like “D & D” than “D&D”. Hence the attempts with weird characters (D𝛂ͦD, D꯴D, D𐓯D...). Unfortunately, some OSs (notably Windows 10) incorrectly draw most combining diacritics offset to the right of the base letter as a zero-width character.
There is a pair of single (non-combining) characters I can demonstrate it with: a combination of K + combining comma below is supposed to look like Ķ but it looks kinda like Қ on Windows. However, Windows stacks combining diacritics so you can create impressively tall Z̷͍̝̿̈́͂̅͝͝͡͡a̷̢̡̢̙̬͕̯̹͍͖̰͕̦͉̘̪͗͂̑͂̑̾́̀̍͗̾́̄̕͝͝l̵̢̨̢̳̻̩̳̘͖͎͎̘̗̬̞͈͌g̸̨͈̠͔̖͎͇̟̍͗̈́̏̿̍̍͗̕o̷͓͎̙͉̯̱̊́͐͘ ̵̣͓̥͈̗̯̹̭̪͕͓̳̈́͌̐̈́̓̈́͑̌̚͡͝t̶̨̛̩̭̬̲͓̣͔̪̠̙̞͚̒͒̽͟͝ͅe̵̛̘͈̜͖̅͊́̅̌̇̒̐͑̚͠͝x̴͌͗͐͌̏͛̕͡͝ͅt̴̡̤̻̺̣̥̝̤̼̺̦̣͎̟́̊̂̈́̃͑̈́̈̈́ while some other platforms just print them over each other like a typewriter does.