I've bred several turtles at their spawn beach with each other until one of two got taller (eggs in inventory). I made sure that there is at least a 10x10 area covered in sand at their spawns before breeding them with sea grass. After waiting for 5 ingame days they wondered of into the ocean and swam in circles In completely random areas. The turtles which I've bred had clearly still eggs in their inventory. At this point I've tried to punch them, to push them back to the spawn location and to rebreed but nothing worked.

So I ultimately think It has to be a bug because I've tried this with 8 different groups of freshly spawned turtles at their home beaches that were all covered in sand and more than big enough.