My sandwich was just fine last time I ate it.

I've figured it out, I think.

The reason behind it is that I always use the taunt key to activate the Sandvich, and one of HH1's plugins removes the weapon switch lock during taunts. As a result, if you "taunt" with the sandvich instead of "firing" it, the sandvich vanishes immediatly and you end up with this:
http://localhostr.com/files/XqfykCa/sandvichtest.dem

"Firing" the sandvich makes it heal to full before disappearing, as intended. Just amusing trivia I guess.