Aha, not sure if this is "correct" behavior on part of Xastir, but it does seem to be intentional. Xastir is treating the packet as a THIRD PARTY packet. The format is described here: http://www.aprs.org/APRS-docs/PROTOCOL.TXT - just search for THIRD PARTY FORMAT. FYI, the code that does this in Xastir is here: https://github.com/Xastir/Xastir/blob/8e065909a2edbefe98e5bcf2cfcce5272f27874b/src/main.c#L13518 The comments discuss making a determination as to treat the packet as "first-person" or "third-party"...
I am encountering this issue as well. I have the additional information that Direwolf (the TNC I am using for Xastir) is printing this out as well, so it is almost certainly being added by Xastir. Note that other people are encountering this as well: https://s55ma.radioamater.si/2017/07/27/send-aprs-datatelemetry-via-xastir-command-line/