What I’m thinking now, how easy is it to remove these parts from the platform? Maybe the Z-offset is wrong for this particular tank. You could play with it by changing it in the printer, but it is not a good solution.
the only thing I can think of that might explain it is if the rubber layer at the bottom of the tank has some kind of dust mixed in with the rubber that would disperse the laser as it passed thru.
If the layer at the bottom was cloudy or had dust mixed in you should see this back in the rest of the print as well. This is not what @DieselPower is saying.