When having a property like property vector2d test: Qt.vector2d(7 / Constants.width, 7 / Constants.height) and selecting a different item (e.g. dummy in the example below) then re-selecting the item with the test property the dynamic property logic will remove the expression and only leave the numbers in like property vector2d test: Qt.vector2d(7, 7).

This is due to updateProxyValues() in DynamicPropertiesSection.qml:321 doesn't care about expressions. The actual expression
Qt.vector2d(7 / Constants.width, 7 / Constants.height)
is reduced to
7 / Constants.width, 7 / Constants.height
and then split on ",". This will result in an array
[7 / Constants.width, 7 / Constants.height]
Each of the elements will be parsed with parseFloat() which removes all the unknown strings and results in [7, 7].

Rectangle {
    id: rectangle
    width: Constants.width
    height: Constants.height

    color: Constants.backgroundColor

    // Qt.vector2d(1 / Constants.width, 1 / Constants.height)
    // Qt.vector2d(7 / Constants.width, 7 / Constants.height)
    property vector2d test: Qt.vector2d(7, 7)

    property real textX: 1 / Constants.width

    Item {
        id: dummy
    }
}