Templates and empty subparts

Yesterday I noticed a strange behaviour in TYPO3′s template handling that led to a problem that I couldn’t fix until today: For some (so far unknown) reason, a single subpart could not be replaced with dynamic content even if it was set to TEXT and value “foo”. The div that was about to be filled dynamically just wasn’t replaced at all. The output showed:

<div id="mysubpart">
  <!-- begin ###mysubpart### --><!-- end ###mysubpart### -->
</div>

So the template autoparser was able to place the marks where my dynamic stuff should go in. After long thinking, I found the difference between this DIV and the others that worked fine: It was the only one that was empty in the template:

<div id="mysubpart"></div>

Changed it to:

<div id="mysubpart">&nbsp;</div>

And now it works fine. Why ever.