Interesting i've never seen a gib key shear off ,would like to see photos.
When installing keys a old fitter at saw mill showed me how back in 1970 ,filing taper face of key to take high spots off which show up when key is pushed into flywheel or pully on shaft with a coating of engineering blue on taper and draw filing so as not to round the face,blue or black marker pen also works, check bottom and sides for high spots on a piece of glass ,also keyways.
New file +file card (special wire brush for cleaning files) when no high spots left and taper shoes touching along whole length and with ,clean +key ways and install ,google for better description.
Peter