Oh my freaking God bro... I just repeatedly tested it with Captain Canon, and it has absolutely nothing to do with a glitch. Don't change your argument from one character to the next, because I am extraordinarily aware that PVZ has glitches and bugs across the board, but you are LITERALLY confusing a fundamental third person shooter mechanic with a glitch, and it is simply NOT a glitch with Captain Canon.
As far as the Flower goes, yes, I've seen very rare situations where the Sunbeam might LOOK like it is slightly going through a wall (but might not actually be doing so), yet that is not the same thing as what you're talking about with Captain Canon and firing from behind cover on a right angle... It's just not, buddy.