In your ending screen there should be only one candle turned off, if there are more than one turned off then the letter won't spawn. If you do have all the endings, for some reason the game has glitched or something, because I tried to do the same thing you're telling me you did but I can't replicate the bug :(. I'm really sorry that this happened to you, if you have all the candles except for the last one I would recommend that you do a new run for the Salazar House story, and try to get the baby ending and see if the letter spawns. Please keep me posted, you can send me a twitter message https://twitter.com/Maldo19 and I'll get to you faster to see what's going on.