.pragma library ensures that different qml documents can access the same data in a .js document. This does not see to work when using qmlcachegen (via CONFIG+=qtquickcompiler) of Qt 5.11.
The attached application generates the following correct output when not using qmlcache:

qml: A.qml set the string: Foo
qml: B.qml appended a string: FooBar
qml: main.qml read the result: FooBar

When building with qmlcache, the output looks like this:

qml: A.qml set the string: Foo
qml: B.qml appended a string: undefinedBar
qml: main.qml read the result: undefined

This issue resembles QTBUG-42102 (where the attachment comes from) showed similar symptoms with the original qtquickcompiler. The difference is, that QTBUG-42102 can be solved by using the right path prefix (qrc:/ vs. qrc://). In this case here, I did not find a solution, yet.