BPM functional debugging

I have a very trivial BPM (MD) that checks the membership of a group and prevent a row update if the user is not a member. It is built using the graphical designer rather than my own C#
It was working in E10 and migrated to E11 without any conversion errors.

However it doesnt work now. I have added an email step to see if the BPM was firing and it is.

I want to debug the code around the group membership check - can someone tell me the best way to do this??

thanks,