
That is a SOURCE lava node, just sitting there on top of a stone node and not flowing at all.
Of course, I couldn't resist digging out the node beneath it, and at that point if flowed all over the place just like normal.
So, I figured one small flow glitch and ignored it. BUT, after some more digging, I opened up another lava cave, and ran into this:


I'm going to go out on the limb and guess that this is a BUG. :)
installed:
minetest_201405112051-0~2747~ubuntu12.04.1_i386.deb
So its a fairly recent version. No map generation mods of any kind installed.