I finally figured it out. It's not a bug, it's supposed to be a feature. To reinforce this, the suspects even have a dialogue in-game.

I was redoing 23mbps today (in the hopes of finally S-ranking it) and was climbing the backstairs when a suspect casually strolled outside of the suspect's mother's apartment. I called out to him to surrender but he retreated back inside shouting, and I quote, "Shoot him! Shoot him through the wall!" (I wish I'd taken a screenshot or recording of this but I have denied write permissions in my recording folder so as to improve performance and screenshotting wasn't an option with an active shooter nearby at the time).

I think the devs did this on purpose to make the AI feel smarter than in any other game. Other games' AI don't shoot you if their visual is lost on you, which from a realism point of view also feels like the enemy AI absolutely never realises that they can hit us through certain objects like wood planks and thus, seem dumb because us players can abuse this to our heart's content and the AI never reacts to that. The logic here is that the devs wanted to imbue the AI (both friendly and enemy) in RoN with an awareness of the facts that:

1. Bullets can penetrate surfaces

2. Once they have even a vague idea of where you may be (through your footsteps or any other sound you might have made), they'll fire in the general direction of the source of the sound in hopes of hitting us.

A combination of these two things is probably supposed to make AI feel more human-like as if in a PvP match.