This isn't a bug. When the server restarts, plugins are unloaded. When this plugin unloads, it automatically removes the stashes attached to cannons. That's intentional because you wouldn't want leftover stashes if the plugin is removed
When a stash is removed, any ammo inside it is dropped to the ground. Those items belong to players and were earned by them, so the plugin does not delete them. It simply drops them instead of wiping them
The same behavior happens if a cannon is destroyed. The stash is removed and its contents are dropped