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Yes it does, because that's what the default Joomla! behaviour is. The root asset node always has lft=1. What was introduced in 2.5.12 is a bug in the PHP code that handles assets. It cocks up and tries to find a root asset node with lft=0, but only if you have multiple nodes with level=0. I don't know how the latter happens. It seems that most sites don't experience that, which makes it extra difficult from me to convince people that it is really a bug and not just a broken site. So, my final thought is that we have yet another bug in Joomla! we can't fix.

Yeah, I know.

I think I've found the problem. It seems to have been caused at some point by ACL Manager. On my site I have half a dozen old articles which didn't have an asset when I imported them from K2. ACL Manager fixed that for me, creating new assets which had parent_id = 0. These were triggering the bug* in Joomla!. I changed the parent_id to the asset ID of com_content and that fixed the problem I had. I can't be sure that it won't happen again the next time I run into asset-less content and try fixing it with ACL Manager. So here's my really final thought: Joomla!'s asset tracking is broken by design.

* Whether it's a bug it's debatable. This is how it's been implemented in the JTableNested since Joomla! 1.6, even though it is NOT how it was implemented in the actual database. I am still not sure what did change in Joomla! 2.5.12 and I'm not too keen to find out. I lost way too much time on this already!