From the Changelog:

In API mode, when you get a function from a C library by writing fn = lib.myfunc, you get an object of a special type for performance reasons, instead of a <cdata ‘C-function-type’>. Before version 1.17 you could only call such objects. You could write ffi.addressof(lib, “myfunc”) in order to get a real object, based on the idea that in these cases in C you’d usually write &myfunc instead of myfunc. In version 1.17, the special object lib.myfunc can now be passed in many places where CFFI expects a regular object. For example, you can now pass it as a callback to a C function call, or write it inside a C structure field of the correct pointer-to-function type, or use ffi.cast() or ffi.typeof() on it.

This sounds like it might need some changes to the stubs to accept this special object.