A strange problem about run_thermal_sail

[jianboqi]

Hello,
I am using prosail for calculating brightness temperature (BT), I found a strange problem that the BT in hotspot direction is lower than other directions, I am not sure if this is a problem of prosail or my wrong use of prosail, could help to check?
My code:

import prosail
import matplotlib.pyplot as plt

emisVeg = 0.99
emisSoil = 0.97
T_Veg = 299.65
T_Soil = 306.65
LAI = 3
hot_spot = 0.03
solar_zenith = 30

LIDF = 57 #spherical
T_VegSunlit = 296.65
T_SoilSunlit = 320.65
T_atm = 8

relativeAzi = 180
viewZeniths = range(60,0,-1)
bts = []
for viewZenith in viewZeniths:
re = prosail.run_thermal_sail(10,T_Veg,T_Soil,T_VegSunlit,T_SoilSunlit, T_atm, LAI,
LIDF,hot_spot,solar_zenith,viewZenith, relativeAzi, emv=emisVeg, ems=emisSoil)
bts.append(re[1])

relativeAzi = 0
viewZeniths = range(0,61,1)
for viewZenith in viewZeniths:
re = prosail.run_thermal_sail(10,T_Veg,T_Soil,T_VegSunlit,T_SoilSunlit, T_atm, LAI,
LIDF, hot_spot,solar_zenith,viewZenith, relativeAzi, emv=emisVeg, ems=emisSoil)
bts.append(re[1])

viewZeniths = [i for i in range(-60,0,1)]

viewZeniths += [i for i in range(0,61,1)]

plt.plot(viewZeniths, bts)
plt.show()