Blasphemy does not work in groups

I play support with Blasphemy. In solo play, I can clearly see that it works; I apply slowing to monsters, which is very easy to see, in addition to the logo above them and at the top of the screen below their health bar.
In a group, it works extremely rarely.

I've tested it with several groups, and also with a friend in a two-player game.
With my friend, it worked once, then stopped working even though we didn't change anything.
Maybe it's not a bug and I'm just really unlucky, but I didn't have this type of bug in the previous season.