I’m 99% sure this is a bug, but I haven’t figured out why it happened or how to reproduce it. Unfortunately I didn’t take screenshots either… but in the “open” tab of the sidebar, I tried to drag a tiddler to the top of the story list, and instead it showed up as:
<a class=\"tc-tiddlylink tc-tiddlylink-shadow\" href=\"#%24%3A%2Fcore%2Fui%2FEditTemplate%2Ffields\" style=\"box-sizing: border-box; text-decoration: none; font-weight: bold; color: rgba(255, 255, 255, 0.54); user-select: inherit; font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "MS Pゴシック", "MS PGothic", メイリオ, Meiryo; font-size: 14px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(50, 50, 50);\">$:/core/ui/EditTemplate/fields</a>
(I got the above from the History List tiddler.)
When I clicked it, it opened a (missing) tiddler with that title. After messing a bit more with dragging, dropping, and closing, I noticed that the tiddler I had been trying to move was listed twice, though after dragging it again or maybe opening it (I don’t remember), the duplicate was removed.
Since I cannot reproduce it, I have not opened an issue on GitHub; I figured someone who knows the inner workings of TW better than I might be able to know what happened if I simply described it here.
(TW version: 5.3.3; browser: TiddlyDesktop; currently installed plugins: Core, Dynaview, Highlight, Internals, JSZip, KaTeX, MenuBar, XLSX Utils)