Why particles are illuminated with a square shape with spot lights?

In other shaders it works fine, it illuminates a rounded area as a spot light should do but when try to illuminate particles it illuminates a squared area, if i increase spot light angle the illuminated square is bigger. Tried using a circle cookie but keeps it squared shaped.
It worked fine with previous versions of Unity, with latest versions it doesnt work.

 

I tried with previous Unity version where i thought it worked before but it doesnt work either. It worked in a different pc i had about 6 years old but not with this new pc, dont know why.