Could you provide an example of this?

Let's assume we have a template "Template:Key":
http://pastebin.com/d7bkRAYv

And the template "Template:Key/Description":
http://pastebin.com/5PJKStN4

Now, if you look at Template:Key/Description (save twice!) or paste it's code elsewhere on the wiki, the table isn't rendered anymore.

PS: The other Key-Subtemplates featured on the description page just contain this like {{Key|Some text}}

I set up a test site. Live bug view:

http://test.bachsau.com/mediawiki/index.php?title=Template:Bug
http://test.bachsau.com/mediawiki/index.php?title=Template:Bug2

You can also test the bug, as the whole installation is meant for testing, and it doesn't matter which pages you edit.

Well, de-prioritising bugs open since 2012 instead of fixing them is really the best way to solve problems, I think. :/

Bachsau: You moved the priority yourself up from 'normal' to 'high'.
I reverted that as priority is meant to reflect reality and as I have seen no signs here that you plan to work on fixing this ticket. You are very welcome to bump it up to "high" again if you are working on a patch. See
https://www.mediawiki.org/wiki/How_to_become_a_MediaWiki_hacker for more info.

If I'd plan on fixing this myself I hadn't reported it as a bug. Sure I'm a registered developer, but not a core developer, and I don't habe time do to complex work for every possible software.