You click to use the ability.
Rangechecks etc get done, if all passes the ability gets triggered.
Unit switches to accessory weapon.
Unit uses the accessory weapon
If you cancel during the last part (i.e. the cooking for the MK2 grenade) then the ability is already over and the weapon is counted as "used", since it started the use. Cancel in this case means new move / target order or the entity dies, you retreat...
A possible fix could be to keep the ability running while it already performs the accessory weapon action, yet that is extremely hard to do. The unit basically gets access to a new weapon for a single use, but will only switch once it finishes it's current action (reloading or whatever). So if you use the time it takes to use that accessory weapon, then you make it rarer, but you do not eliminate the possibility. For example a Fallschirm has a 3+ second reload time, meaning it could take a while till he uses that weapon. If you make it too long, then people can use the ability, wait for the weapon to trigger and cancel the ability to do not make the ability use count. If you are playing a lot, you should know that this could be done with few abilities, if you knew how
