fill-replace-air does not work for the block above a water lake

Using the fill command to replace air blocks directly above a water lake does not work unless you break a block at that location
Command used:

fill ~ ~-1 ~ ~7 ~-1 ~7 grass_block replace air

Expected behavior: cover the lake that I'm standing next to (at the appropriate corner, yes) with grass blocks
Actual behavior: Only a few specific blocks were replaced; these were spots where I had broken a grass block manually