From what I can think of, its coming from 2 different source ip’s.
One could be your management ip and other your self IP address.
When the traffic comes to the forwarders, it does reverse lookup for the IP and creates the log file respectively.
But I dont see a problem on this, its quite common. All you have to do is, work with your splunk team, to index them properly. As long as both the logs source type are same, and indexed to one common indexer, its not a big deal.
Else you’ll have to make changes on the LTM to force the logs to go out through one interface, either mgmt or tmm. There’s KB articles to that.