"Cannot assign object to list property "data"" when declaring QML instance of C++ item declared in lib

I get the following error when running the attached application:

qrc:/qml/main.qml:14 Cannot assign object to list property "data"

So far, I've only seen the error with a static build (reproduced with separate debug and release builds) of Qt 5.12. I tested with a dynamic debug build of Qt 5.12 and it runs fine.

I would attach a qmake version of the project, but I don't know how to replicate it easily.