Waterlogged slabs lose water after leaving server and joining back

On the cubecraft sever in skyblock every time i create a waterlogged block with wood slabs or cobbles slabs after i leave the server and come back the water is gone and its just the slab. Everything is drying up around it as well so i know the water is gone. I have a 1 block deep hole add water then place slab in the bottom half of hole. I have also tried putting the slabs in the top half same thing no water when i come back to my skyblock world