@JStrodt_WMDE Does this problem change the output of the template, or is it a case of not following a wikitext convention?

@Deskana It's a case of not following a wikitext convention. The problem that arises out of this is that it makes the wikitext clunky and it confuses users of the wikitext editor, who then start correcting the wikitext to set it right.

As far as I understand (and already wrote in the discussion on German Wikipedia) this is a duplicate of T138492: Add support for actual format strings to TemplateData's "format" parameter. While TemplateData has a custom format, Parsoid currently only understands inline and block, and uses inline for all unknown formats. So instead of the specified format {{_ |_=_}} the default {{_|_=_}} is used and the spaces are removed.