I just want to remind you of a few old uncorrected bugs.
the Fish Delivery point always counts the number of fish in the basin as zero.
just build a fish delivery point, spawn some fish and see:
for this reason, the "Max Critters" limit does not work properly - duplicants will always bring new fish, no matter how many fish are already in this basin.
let's look at the code
int cell = Grid.PosToCell(this);
it corresponds to this part of the fish delivery point, which acts as a solid block and is not part of any room, so the fish delivery point cannot correctly calculate the number of fish.
I suggest changing the code a bit like this, and then everything will work out properly.
int cell = Grid.OffsetCell(Grid.PosToCell(this), spawnOffset);
Steps to Reproduce
This issue has not been confirmed by a developer yet.