Assistant doesn't register the qch file listed in the qhcp

I'm having trouble with assistant not loading the compressed help file when started in "-collectionFile" mode.

Attached are the files which can reproduce this problem. I run:

qcollectiongenerator polyvox.qhcp -o polyvox.qhc

to create the collection file. This references the polyvox.qch file in the <register> block and so upon doing:

assistant -collectionFile polyvox.qhc

Assistant should load and display the documentation. However, there is nothing shown in the "Contents" list at the left, there's one very thin tab shown next to the search tab with no text in it and pressing "home" gives

The page could not be found 'qthelp://org.thermite3d.polyvox/api/index.html'

I know that the compressed help file is valid since I can go to Edit → Preferences → Documentation and add it there. It all then works until next time I open Assistant.

When running assistant, there is no error message given about not being able to find polyvox.qch so I don't know where to start looking for a solution. All I can guess is that this is a caching problem of some kind but I can't find where the cache is to clear it.