This has been fixed in my po branch.
+> My commit dcd57dd5c9f3265bb7a78a5696b90976698c43aa updates the
+> bugfix in a much more elegant manner. Its main disadvantage is to
+> add an (optional) argument to IkiWiki::filter. Please review.
+
-- [[intrigeri]]
+>> Hmm. Don't like adding a fourth positional parameter to that (or
+>> any really) function.
+>>
+>> I think it's quite possible that some of the directives that are
+>> calling filter do so unnecessarily. For example, conditional,
+>> cutpaste, more, and toggle each re-filter text that comes from the
+>> page and so has already been filtered. They could probably drop
+>> the filtering. template likewise does not need to filter the
+>> parameters passed into it. Does it need to filter the template output?
+>> Well, it allows the (deprecated) embed plugin to work on template
+>> content, but that's about it.
+>>
+>> Note also that the only other plugin to provide a filter, txt,
+>> could also run into similar problems as po has, in theory (it looks at
+>> the page parameter and assumes the content is for the whole page).
+>>
+>> [[!template id=gitbranch branch=origin/filter-full author="[[joey]]"]]
+>> So, I've made a filter-full branch, where I attempt to fix this
+>> by avoiding unnecessary filtering. Can you check it and merge it into
+>> your po branch and remove your other workarounds so I can merge?
+>> --[[Joey]]
+
[[!tag patch]]