I believe I have placed water in such corners many times and it always merged and filled the space in a square, but what is this?!

Does it have anything to do with me first placing the center sources (those one level below)?

I have seen water not spreading before, but only in caves where the water was flowing from top, believing it is some loading/generation glitch. I have seen some strange water flows when trying to mine obsidian - sometimes the water just chooses to flow in one direction, but I really do not see any pattern here (only suspecting that it chose to flow towards the lower water-sources in the centers of the edges of the square, when I started placing the upper corner sources). Is there some preferrence to flow towards something instead of spreading?

I have removed all the water sources, put stone in the edge-centers and placed single water source and this is the result (makes no sense to me):

That is single watter source (left-top), I trully have no idea what is going on.