I had the faintest epiphany here:
I'm reminded of the very often used trope, especially in cute slice-of-life manga, where at a sleep over, someone has someone else bring them to the bathroom in the dark, because they're scared or don't want to be alone. There's even the condition that they have to stay right by the door.
I cannot imagine that in this circumstance, you wouldn't hear the business of whoever's beyond the door. Obviously, this manga takes a different context than those but realistically, I don't think a big deal is made during those moments.
I feel like this "hearing other person on the toilet" thing is just a punchline that is, somehow, being taken disproportionately. It's not a big a deal except for Hino, at that moment. Then, as demonstrated, she got over it by the next chapter.
Just my two cents.