/placeblock lava may create lava that does not flow

There are other glitches that have been reported about lava, but this seems to be a new issue.

I'm playing in creative with cheats, and I was using /setblock ~ ~ ~ minecraft:lava to create a square ring of lava. I noticed that sometimes the lava did not flow downwards. There IS lava there (because it can be removed with a bucket, and additional lava can't be placed there), but reloading DOES NOT fix this.
I can upload my world if necessary (although I'm building on a superflat world with only a layer of air and I built a bunch of other stuff in the world a little while away. This means I'm building directly over the void).

EDIT: After more experimenting, it seems that any lone lava that has been /setblock'd will not flow unless another block (even another lava that was placed with /setblock) is placed directly adjacent. Note that diagonally does not count. I suspect that water may also be affected.