Vue prop warnings

[MattApril]

Warnings started appearing after a recent Vue update:

console.warn
[Vue warn]: Wrong type passed as event handler to onLeaveCancelled - did you forget @ or : in front of your prop?
Expected function or array of functions, received type string. 
  at <Transition name="collapse" onBeforeAppear=fn<bound beforeAppear> onAppear=fn<bound appear>  ... > 
  at <CollapseTransition onAfterEnter=fn<bound afterFiltersExpand> onEnterCancelled=fn<bound cancelledFiltersExpand> >