There’s a bunch of non-obvious hazards which exist around cutting into containers which have held fuels, including but not limited to:
An empty propane container could have an gas+air mixture inside it in proportions which could explode.
Any liquid residue in an empty fuel container can be vapourised by the heat of the cutting, creating a fuel+air mix which can spontaneously explode from the pressure and heat, even without sparks (think: diesel engine principle but instead of a 0.25L cylinder it’s a 200L cylinder).
An empty gas canister can be contaminated with heavier oils/waxes that could build up over the years of refilling cycles, creating the same hazard as noted for empty liquid fuel containers.
Methods to reduce the hazard include:
Thoroughly washing the inside of the container before cutting (access can be difficult though).
Filling with water and cutting while full of water. (Container musn’t be sealed, and the cutting method must be safe to use in a wet environment, and further hazards such as introducing oxygen into the container need to be considered).