The Bode Phase plot which you got is correct because the transfer function is written as
When you calculate phase angle then
For (1+jw) the phase angle is= -atan(w)
For (1+(-1)jw) the phase angle is which is -atan(-w) written as atan(w)-180
So, the overall phase angle becomes ɸ =-atan(w) + atan(w)-180
ɸ = -180
So, the Phase plot you will get at -180 deg.Here atan is nothing but inverse of tan.