In my anecdotal experience, changes in how much of your neighbor's mail you get often ends up being due to a change to which individual carrier is delivering to your address. For over a decade growing up, we got our mail delivered by an exceptionally chill guy named Bill. As kids we'd get excited and run up when we saw him because he'd chat and joke with us, and he knew my parents by name and would chat with them often as well. We never got anyone else's mail from him, and no one ever got ours. Eventually, Bill got transferred to a different route, and the new person who delivered on our street would inexplicably stuff as many letters as he could into the insides of magazines going to a given address, and there was a high rate of getting our neighbors' mail inside of our magazines. When this first happened, my mother tried bring this up with him nicely, but nothing ended up changing. I can't remember how receptive he was to the feedback, but my mother didn't try to bring it up with him more than maybe once after that because she could tell it was a lost cause, and whenever it happened she'd roll her eyes and say that Bill would never make a mistake like that.
After college, I lived in an apartment for over four years where apparently the woman who had previously been in it switched to a different apartment in the same building (which was quite large, so I don't think I ever met her). In the first couple of weeks, we for a couple pieces of mail of hers that we'd leave with the doorman, and he'd give it to her (or maybe have it put in her mailbox instead), and this stopped happening after that for a while. A few years later, we started getting some mail for her again out of nowhere, and the first time I brought it down to the doorman, he mentioned that the person delivering mail to our building had switched recently. I have no clue why someone who hadn't delivered to the building before would be inconsistently delivering mail to her old address, but it basically never stopped happening during my remaining time there.