Note: this bug affects ANY page that currently contains some '<' including many Lua sample code between {{{triple braces}}}.
The page is parsed when saving, anything that looks like a possible HTML tag will cause an exception where the whole text is lost. Then ALL HTML-escapes like "<" or numeric character entities are converted to plain-text literal characters (not generating any HTML). The wiki then stores and displays the result of this parsing.
Something changed recently on the code implementing the wiki, which caused incompatible behavior.
A very large number of wiki pages are affected by this new bug. Be careful. The wiki implementation has a serious bug, at least it should not allow saving the page if there are '<' but should inform the user that he must fix the content, or the wiki should first HTML-encode any "&" found anywhere into "&", and then any "<" found anywhere into "<"
Visibly the HTML-escaping of the input text was removed or forgotten in the new implementation!