Hi Tammy,
I think when you use "Login user" attribute it gets the "audit_userid".
I checked in SDM 17 and "Login user" is an attribute added in this case to Activity Association List.
If we check audit_userid definition, we can see that is a local value that means is for store data provisionally.
audit_userid LOCAL SREL -> cnt.id TENANCY_UNRESTRICTED
In your case you need add in your Activity Association List the attribute "Last Modified By"(last_mod_by).
last_mod_by SREL -> cnt.id TENANCY_UNRESTRICTED
And use this attribute in your site-defined condition as "Last Modified By" Does Not Equal "Affected End User".
Regards,
Carlos