I walked trough the kde-workspace-bin debian source, and seems like PowerDevil launches the upower service via DBus if it is not running, but I was unable to find out why the thread that monitors the battery level change don't work properly in some cases. It looks like some hardware specific information from the battery may cause be the issue, i.e., when upower starts from PowerDevil, it may not work as expected and the thread don't start properly. The weirdiest thing is that the battery icon *does* work, i.e., it shows the proper level; only the low/critical actions aren't triggered.
I do agree, however, that the issue seems to be hardware specific, since it don't happen with most users, but it should be put in a wiki/errata so people can actually find the workaround for it.