Qbs fails to detect property changes/invalid properties after building once

Qbs fails to detect invalid properties after (partially) building once.

See the following example (using cpp.cFlagsx as invalid property):

> rm rf qtc_Desktop*

> qbs build profile:qtc_Desktop release cpp.cFlagsx:-Werror
Resolving project for configuration qtc_Desktop-release
ERROR: Unknown property: cpp.cFlagsx

> qbs build profile:qtc_Desktop release cpp.cFlags:-Werror
Resolving project for configuration qtc_Desktop-release
Setting up build graph for configuration qtc_Desktop-release
Building for configuration qtc_Desktop-release
Applying moc rules for 'qtcreator'.
Applying moc rules for 'Aggregation'.
ERROR: Build canceled for configuration qtc_Desktop-release.
The following products could not be built for [...]

> qbs build profile:qtc_Desktop release cpp.cFlagsx:-Werror
Restoring build graph from disk
Building for configuration qtc_Desktop-release
generating app_version.h
moc qtlocalpeer.h
moc qtsingleapplication.h
[...]