Backward compability! Backward compability!! Backward compability!!!
Fragment of a discussion from Help talk:Special property Subproperty of
The "Then it changed to ..." sounds very much like a bug. When did this change and was it reported back then? However, it should probably allow both ways with
[[Subproperty of::Property:Superproperty]] as the preferred method. Either way its worth a bug report.
If there is an issue, I wonder why  [[Subproperty of::Area]] in Property:Surface area works. Locally I could not find any irregularities with either setting (with or without namespace).
I have to investigate.