Torben,
Mmmh. I had a look at your test page and, actually, I recognised that you simply copy&pasted the xowiki-doc {{toc ...}} statement into the page. This is somewhat problematic because the string is a auto-generated one meant for documentation purposes, it reflects parameters and the default values (which might be hidden). Nevertheless, the issue with the uneven list is due to a loss of information when using double-quotes for empty-string argument values, e.g.:
{{toc -decoration plain -expand_all 1 -source ""}}
If you write ...
{{toc -decoration plain -expand_all 1 -source {} }}
you should be fine. I rewrote your example at the test page accordingly.
As for the background, whether this is intended or error behaviour, I will check back later with a code crawl. Or, maybe, Gustaf Neumann may shed light on this in the meantime.
//stefan
