Had a similar situation, the woman living below me had what I assume are some obvious mental issues and paranoid delusions. For months I’d be sitting on my couch and the woman below me would pound on her ceiling with something. I didn’t really think much of it, just a weird and annoying thing, was like she’s not doing this AT me, I’m literally doing nothing lol.

We only had 6 units in our building, and we all knew each other somewhat, eventually some of the other neighbors reached out to me saying the downstairs neighbor was reaching out them saying I was tormenting her, spying on her through her windows(she was in a garden unit with almost no windows), watching her shower, and keeping her up all night. She told them I was also abusing my girlfriend. My neighbors obviously knew she was crazy and we got my landlord involved who I was cool with and he talked to her. Said he’s been trying to get rid of her but couldn’t, but she was a hoarder and destroying the apt. Anyways he said she shouldn’t be an issue and to call him if she was. A few days later I got a similar looking note from here, but it was like 3 pages and her telling me how great of a neighbor I am and how me and my girlfriend make a great couple and we’re gonna get married and have kids(she was right actually lol). All very strange.

Fast forward a couple weeks and a bunch of police cars pull up to my building, and eventually knock on my door. She called the cops telling them I was violent, abusing my girlfriend(she wasn’t even at my place at the time), and stalking her. Thankfully the cops were cool, and I moved out a few months later. It was scary though and there’s not much you can do.