Properties are for Guiding, not for Restricting
When you define a property-set with 'strongly-typed' property values, those values are used to provide an intuitive, guiding UI for tagging and locating items.
The actual value does not force a strong relationship to the original property-set's predefined values. This is by design, to not slow-down common repository operations and for keeping artifacts management simple by allowing properties to change and evolve freely over time, without worrying about breaking older property rules.
Properties are therefore a helpful and non-restrictive feature.