From the original bug report:

Adding a property string foo to a component is not available to the QML LS until a) after the modified file has been saved and b) after the code has been built and c) after the enclosing QML file is subsequently edited. That's three steps necessary to let the code model know about a single string property addition.

It seems that typing "property string foo" will be shortly colored correctly via semantic highlighting, and then the entire file will be uncolored. This stays this way until the qml file is modified again, see also attached movie.

The highlighting should not jump around all the time, and it should not fail after the "foo" property was written entirely.