Case Record Type Is 'Null' or Shows as 'Deleted' in the Case History Related List in Salesforce Email-to-Case

Cases created via On-Demand Email-to-Case (a Salesforce feature that automatically converts inbound emails into Cases using routing addresses) are not assigned the appropriate Case record type according to the preset value configured in the Routing Address. Instead, the Case is assigned the default record type of the Queue Creator, which may or may not be 'null'.
As a result, the Case History related list indicates that the Case record type value has been deleted or changed.

Root Cause

This behavior occurs under specific Support Settings configuration conditions. Navigate to Support Settings:

• In Salesforce Classic: Setup | Customize | Cases | Support Settings
• In Lightning Experience: Gear icon | Setup | Platform Tools | Feature Settings | Service | Support Settings

The issue occurs when both of the following conditions are true:

1. The configured Default Case Owner value is a queue.
2. The configured Record Type Setting value is set to: "Override the existing record type with the assignee's default record type".

When these two settings are active together, Salesforce assigns the default record type of the Queue Creator to the Case, overriding the preset record type from the Routing Address.