The common mark spec says fences may only be indented three spaces but you have indented it four, looks like it missed the close fence indented four spaces so the fence runs to end of file.

But of course it also depends on the version of markdown the highlighter supports.

The highlighting in Geany is provided by the lexers in the [Scintilla](https://www.scintilla.org/) project, so you need to report it there to discuss which version(s) of Markdown it supports. If you can, check first how it works in Scite, the reference editor for Scintilla since its version is ahead of Geany, although the release notes suggest not much has changed for the Markdown lexer.

Please put a link to the Scintilla report here.

@nomadbyte remember that everything in the `scintilla` directory is imported from the [Scintilla](https://www.scintilla.org/) project which is a separate project from Geany and is used in many other editors/IDEs. So although its a reference, its not necessarily the latest version of the lexer, and changes need to be made in Scintilla first and against the version there (parenthetical hint, the structure of Scintilla has changed markedly lately, and Geany has not followed as we wait for it to stabilise, so the lexer versions in Geany may be somewhat behind).