In https://codereview.qt-project.org/c/qt/qtcharts/+/330692 it was discovered that qtbase doesn't set the INTERFACE_QT_COORD_TYPE property on the Core target. It only gets added to the Qt6::Core target by the Qt6CoreConfig.cmake file and only when QT_NO_CREATE_TARGETS is not true. This means INTERFACE_QT_COORD_TYPE is only set if you are building against an installed Qt or if you are doing a per-repo build against an already-built qtbase. It is NOT set if your are doing a top level superbuild or building just qtbase. It is also not set if building examples.
The target's COMPATIBLE_INTERFACE_STRING also needs to have QT_COORD_TYPE added (it is suffering from the same problem and only gets updated for the same cases mentioned above).
The handling of QT_MAJOR_VERSION also seems analogous, but that one seems a bit more complicated and should be addressed in its own separate issue (it interacts with autogen).