Aquifers down below Y0 can be a combination of water and lava aquifers instead of deciding. This is likely due to the change that made aquifers more likely to generate on the same height when close to each other, so two aquifers might be water and lava and will merge together looking like one aquifer with lava and water.

This is an issue because aquifers merging is meant to give the illusion of one big aquifer, not multiple ones together.