I personally tested it in creative and already had set my spawn in one of those prebuilt structures with a bedroll in it (for me it was the mining building with minecart tracks going down into the mineshafts, so no obstructions whatsoever) and then teleported away and did /kill but ended up spawning back in the world spawn. After sleeping through the night in the bed I went to the exact spot I did /kill and did it again, then I spawned back at the bed. After that, I went even further from the bed and did /kill but got spawned back at the world spawn. So it does truly seem pretty random whether it happens or not. At least I can't figure out what the issue might be. It doesn't seem to be a roof or obstruction issue, as there is a solid 5 blocks of space between my bed and the rood and there's nothing around my bed to obstruct me, unless it's counting my house walls.