Been doing it for years. My grandparents did it too. Never had a problem. Not suppose to use the whole box but it does neutralize odors in the drain. Otherwise, not often, only when really bad and liquids for all else.

Prob is people don't maintain their drains. Not in op case, but you know.

Normally people don't maintain properly. But bleach is a better alternative to putting any powdered substance in a drain. it actually eats hair if you let it sit in the trap. Also helps with drain odors. If you have a septic I recommend using very little or no bleach, it will eventually kill all the good bacteria living in your septic tank breaking down the solids. Same goes for antibacterial soaps. Bad for septic tanks.

Jus a thought, could it be coming from the shower drain. Maybe some construction debris in the trap?

 

What you are describing is odd, in my experience.