It seems that if the player disconnects after going into a portal and before loading into the world, the player is not properly moved to the Nether Portal, and instead retains the coordinates they had in the overworld. (For example, due to lag while loading the new dimension.)

This usually causes the player to suffocate deep in a wall before they load in when they disconnect, a situation which also causes gravestone mods to fail, worsening the situation.

This is occurring on FTB Revelation, and as far as I can tell, this mod is the culprit.