"Could a percussion fuse be unstable???"
Possibly, but unlikely, and far more likely that an unexploded one would go off if dropped on its nose if, for some reason, the striker didn't slide forward when the shell impacted.
Apparently the percussion caps used in these things were quite large and had a lot of fulminate in them, and I have no clue how mercury fulminate ages over time.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.