None of the effects will occur during any Challenges.
Resurrection items activate in a set order. Missing Poster has the lowest priority of all / the second lowest priority of all, with only Tainted Lost's Birthright effect being lower.
If another resurrection item activates before Missing Poster while in the sacrifice room, then the player will not become The Lost, but The Lost will still be unlocked. Missing Poster will also not get used up by this.
The Lost's intrinsic Holy Mantle is not gained when revived into this way.
If unlocked, The Lost will have his intrinsic Holy Mantle when revived into this way.
Dying to Sacrifice Roomspikes specifically is not required to unlock The Lost and any death counts.
If Isaac teleports out of the room before dying, i.e. is teleported by the spikes to Dark Room, The Lost will not be unlocked.
The first scenes in Ending 15 and Ending 17 show The Missing Poster pinned to a utility pole, with a silhouette of Isaac's mother who seems to be searching for Isaac in the background.
There are many puzzle pieces in total. Assembling all of the jigsaw puzzles and figuring out what they meant was initially designed to be a collaborative effort of the community. As such, Edmund and Tyrone had openly expressed disappointment towards the datamining.[1]
The unlock pop-up of Missing Poster calls it "Lost Poster". It is unknown if this is a mistake or a hint.
A chest always containing the Missing Poster will appear as a room clear reward from the Dogma room, although it is impossible to access it without the use of console commands.