Okay, so I'm not sure if this actually exists or if it was a weird dream I had but it was a Madoka Magica doujin that was about how Homura was getting letters from Madoka that were addressed to Sayaka. She is confused about why the letters were being sent to her but then she finds out that Sayaka had lived in her apartment before she had killed herself. She then decides to write back to Madoka as if she was Sayaka.

I am like 90% sure that this is a real thing that exists but... Who knows.

Alright so I actually read something like this about two weeks ago, except it wasn’t a doujin it was a fanficion.
I’m sure this is what you were looking for, but I could be wrong.

https://m.fanfiction.net/s/10793135/6/Flicker