SunCat, sorry I mised that comment. 😃
I think that is because minecraft destory that block and then setted air but there was already air. ( air is placed if block is destroyed )
if so maybe this can be fixed by testing if block is "air" and mode is "destroy" then only calculate size and no setblocking / filling. 😃
sorry for my bad English I am Czech.
but i don't thinked triple chest bug, i thinked texture bug.