The way to reproduce:

1) Make a pond like screenshot 1.

2) Collect one of the two source blocks at the far left, wait some seconds, and collect the other. The pond still looks the same.

3) Do the same as 2), note that this time collect the second source block before the block the first source block was in is re-filled with water.

4) Notice that those two blocks contain non-source water blocks, like screenshot 2.

Note: If this pond has only one column (like screenshot 3), then collecting the source block at the far left immediately triggers it to be a non-source block (like screenshot 4). Don't know if that's intended.