Are you sure you are building qwt against your Qt6 installation? Can you successfully use qwt in a project built with QtCreator? (I know these are "Have you checked to see if it is plugged in?" questions, but I've done things like this in the past, where I have had mismatched builds and nothing seems to work).

It is MOC that results in all of the QMetaType references so something could be wrong with your qwt build.