After mating, turtles are supposed to travel back to their home beach, start digging and lay their eggs, with the home beach being a surface made of sand or red sand (#sand tag). As of 1.16-pre4 or 5, the home beach detection looks one block too deep, which results in one block thick surfaces of sand, with any other block underneath, no longer being detected as a home beach. This breaks a lot of turtle farms.

If the top surface is for example made of stone, and the layer right below it made of sand, turtles start digging and lay their eggs just fine, see screenshot.