The event itself is probably using the IP address for the Host value. It might be helpful to see what Splunk has indexed for the host value of that event, as well as what is the extracted field value. To compare, run a search similar to the following:
| metadata type=hosts | search host=<your_ip_address>
| metadata type=hosts | search host=<your_fqdn>
index=your_index source=udp:514 | fields host
The value from the metadata searches are what is stored in the index, and the values from the search with | fields is likely the extracted value.
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