The portal is not populating values in a Custom Picker for a service account.

Hi Appian Experts,

Need you help in an issue that I am facing in the Portal.

We have one Portal UI which is having a Custom Picker field which is extracting/suggesting the values based on the input given by the user from the Record Type which is fetching data from the DB:

The below image is taken when accessing via a personal Admin account on the Interface itself:

When using the Service account(Which is added in the Users group and having Viewer permissions) while accessing the Portal  the custom picker field is showing No Results found:

The below image is taken when accessing the Portal via a Service Account:

We are unable to identify why the list is not populating on the Portal. Although earlier it was working well. Do anybody else faced something like this while using the Portal ? If Yes, then what they did differently.
Need help in this regards.

Thank you for your time and patience!