❯ where.exe qmllint
C:\Qt\6.10.2\msvc2022_64\bin\qmllint.exe

Unknown argument "Assertable" to pragma ValueTypeBehavior [syntax]
pragma ValueTypeBehavior: Addressable, Assertable
^^^^^^

Another attribute to the ValueTypeBehavior pragma is Assertable, introduced in Qt 6.8. Due to a mistake in Qt 6.6 and 6.7 the a as rect above not only checks whether a is a rect but also constructs a rect if a is of a compatible type. This is obviously not what a type assertion should do. Specifying Assertable prevents this behavior and restricts type assertions for value types to only check for the type. You should always specify it if you are going to use value types with as. In any case, if the type assertion for a value type fails, the result is undefined.
valuetypebehavior docs