Plugin is not triggering for Associate Message using WebApi Client side request

Hi
 I'm using D365 online CRM and on premise.
I wrote a plugin step for 'Associate'. When I'm adding records to subgrid, the custom exception is triggered in my code. But when i'm trying to call association using web api, the custom exception is not triggering. As i noticed that in Fiddler adding to subgrid the xml request is forming like below.

https://****.crm8.dynamics.com/AppWebServices/AssociateRecords.asmx

the request is showing error 500 for custom exception.

<?xml version="1.0" encoding="utf-8" ?><soap:Envelope xmlns:soap="schemas.xmlsoap.org/.../envelope" xmlns:xsi="www.w3.org/.../XMLSchema-instance" xmlns:xsd="www.w3.org/.../XMLSchema"><soap:Body><Associate xmlns="schemas.microsoft.com/.../WebServices"><subType></subType><objectType>8</objectType><parentObjectType>10084</parentObjectType><objectId>&#123;XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX&#125;</objectId><parentId>&#123;XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX&#125;</parentId><associationName>gr_gr_sysconfigurations_systemuser</associationName></Associate></soap:Body></soap:Envelope>

The response of exception:

And the web api request is forming like below.

https://****.crm8.dynamics.com/api/data/v8.0/gr_sysconfigurationses(XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX)/gr_gr_sysconfig_systemuser_useraccess/$ref

the sucess response is showing for association using webapi shown below:

{"@odata.id":"https://****.crm8.dynamics.com/api/data/v8.0/systemusers(XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX)"}

Can any one have idea on this issue?.