Sometimes, dupes will be stuck in doors, that are set to open and close automatically. They will be shown as entombed, even though other dupes can walk through the door normally. This can be fixed by set the door to open all the time in the menu and let another dupe set the door to open afterwards. As i play with some DLC-compatible mods enabled, i'm not sure if this is caused by the base game or one of the mods (personally, i think self sealing airlocks is a candidate for being the culprit). I just wanted to mention it here, so that you can take a look, if something in the vanilla game is causing this.
It happens randomly, no reproduction is possible. Sometimes it happens with the first door build in a new game, sometimes it takes hours, where nothing happens.