Light 0 on farmland

On farmland light stats switch to 0 (watch screenshots)

Walking next to farmland block :
2018-07-18_00.53.28.png : light on pressure plate : 8 (5 sky, 8 block)

walking on the border of the block :
2018-07-18_00.53.36.png : real light on farmland : 7 (5sky, 7 block)

walking in the center of the block :
2018-07-18_00.53.37.png : in fact, light on farmland : 0 (0 sky, 0 block)

So i cannot pant anything