Web services calls executed through custom plugin failing HTTP Authentication with SSO enabled

We have enabled SSO (through SAML) on our Appian on-premise environment. We also have restricted the SAML Authentication to Specific Group.

The issue we have come across is that the Web Service calls are failing most probably due to unauthorized users.

I would like to note, that all these Web Service calls are executed through a Custom Plugin Servlets (Synchronous Process Model Web Service, downloaded from Appian’s Shared Components) in order for the Web Service Response to contain data of all PVs from the process instance that was executed. The code in the plugin servlet is never executed because the user is not authenticated in Appian (there is a redirection to the ADFS login page).

The Web services are invoked from an Appian system admin user account, and this account is NOT part of the restricted SAML group. The issue has to do specifically with Custom Servlet Plugins that used to invoke process models exposed as web services.

Any suggestions on how to proceed further?