Since some rich text editors like tinyMCE strip out all non-valid HTML, is removed. It's pretty trivial to also support [equation] with the same options.
Change drutex_reder.inc line 74 to:
'pattern' => '@[\[<]equation( [^>]*)?[>\]](.+?)[\[<]/equation[>\]]@se',
in other words, replace the < and > with [\[<] and [\]>]