Duplication detection not working

We have activated duplicate detection and have set Title and Customer as the attributes to compare.
All roles have been added, and the permission level for ticket search operations is set to “System.”
Tickets in the following status are being compared: “All Tickets.”

I have sent multiple identical emails with the subject “test” and the same content. Unfortunately, the system does not recognize them as duplicates.

Am I possibly overlooking something in the configuration?

System: Running von docker with version 6.5.1. Self hosted instance.