Major problem with TNT cannon!

When a TNT cannon is ignited, tnts that go out of sight will decrease their total firepower to
a certain value. For example, when 12 tnts are fired to hit a mob accurately (e.g. Elder guardian) ,(normally only 3 tnts is needed to kill an elder guardian) If I don't follow the 12 TNTs to keep them in sight, no matter how many tnts is fired accurately to hit the mob , it will take 2 more hits of a diamond sword to kill the elder guardian.

THANK YOU VERY MUCH FOR READING THIS.