Water does not despawn/evaporate correctly

I have dispensers placing water, setup on a relatively long delay (for a mob grinder). There are 15 in a row, with a blank spot for the 16th, all facing in on a 16x16 grid.

When the dispenser place water, everything works correctly.

When the dispensers scoop the water, every 5th cycle or so a water spring block randomly appears in the middle of the 16x16 area. This can trigger a pond-effect, where the new spring block works with the dispenser's water placement to create a 4x4 patch of spring blocks, which rapidly fills the entire 16x16 area with spring blocks.

=EDIT=

I have added screenshots of what my issue looks like. This is on my accessible floor, but even with the sparse number of dispenser I have here it's doing the same thing.

A full row of 16 dispensers will fill ("pond") the entire spawning floor. I've tried 15, every-other, different spacing patterns (2 dispensers, 1 cobblestone, 3/1, 3/2, 4/2, 4/3, etc.), trying to get this to stop. Right now each bank (North, E, S, W) dispense all at once. I've tried having caddy-corner sides dispense separately, such that N/S fired together, then after a delay, E/W fired together. I thought there was some interaction with having the 4 sides drain together, so I've put E/W on a 5-repeater delay, so N/W is full flooded and stable before E/W floods, and the same for draining, but I am still getting this issue.

As you can see in the screenshots, the placement of the "phantom" spring block is seemingly random.