Record Type Field Not Visible for External Users When Running a Report

Workaround: Use a Custom Text Field to Capture Record Type

1. Create a custom Text field on the object to capture the Record Type value.

2. Create a Workflow Rule (or Flow) with a Field Update action to populate the custom text field whenever a record is created or edited.

3. After activating the rule, trigger a mass update on existing records to populate the custom field retroactively.

4. Use the custom text field in your Custom Report Type as a filter or column instead of the Record Type field.

Note: Since workflow rules will only fire every time a record is edited or created, once the new rule has been activated, a mass update will need to be triggered all existing records. 

The Record Type Field is available to external users on Standard Report Types. It is not available to external users in cross filters regardless of report type.