Black Powder is such a simple formulation that you can soak it in water for decades then dry it out and it will still go off.
If the granules break down it can cause pressure problems but other wise its still an explosive mixture.
With ancient powder removed from such a cannon the powder could have historical significance and they may wish to analyse it to discover its origin.
There were many sources for BP in those days, with variations in quality of ingrediants. Each batch was tested to determine the exact powder charge needed.