It turns out there’s been a known problem since WordPress 2.1 was released two months ago. Some plugin hooks have changed, and plugins that used to only get called during post editing are also getting called during comment publishing. I grabbed an updated version of the plugin, and it seems okay now.
Oddly, most (but not all) tags survived unscathed during the two months running earlier 2.1 releases. It’s only since moving to 2.1.3 that it’s been consistent. Oh, well, at least it prompted me to find the fix.