It’s possible that this is intended, but I thought I’d list it just in case.
Had a group doing Arah story mode last night for the first time. Some of the party members decided to use the airship cannons on the big mouth monster who had a retaliation buff. In hindsight, this was not a wise move: it caused the cannons to be destroyed. This meant we were unable to kill the attacking dragons in the next event, and we had to leave the dungeon and start over.
If this is not intended, it would be good to make the cannons unusable or invulnerable during that phase. There was no obvious indication we needed the cannons anymore since the airship was docked.