Qt print dialog doesn't allow selecting some page sizes

Printing from either Okular or Gwenview (KDE 5.9), using Gutenprint 5.2.10 or 5.2.12 with the Canon iP7200 driver and corresponding PPD file, I am only able to select a subset of the offered page sizes. Letter, A4, and some others work; others, such as 5x7, cannot be selected (if I attempt to select them, the dialog continues to display Letter or A4 or whatever was last selected).

Non-Qt apps (e. g. GIMP) will select any page size I choose.

I am the Gutenprint project lead; this issue was reported by one of our users, and I reproduced it.