Well, that's really weird! Looking at the code, I'm pretty confident it's not Transverse Jump itself - the location-picking looks correct and is capped to something like a light-year in distance from the current star system.
It's possible that something set the fleet's location right after the jump - this could happen, for example, if the fleet was orbiting something, and the orbit didn't get reset when it went through hyperspace. But since a fleet moves a bit as part of the normal jump transition, that cancels any orbit it might be holding in terrain. Still, let me add a fail-safe here and fleet.setOrbit(null) on jump transition - that seems like the right thing to do in any case. Like TJJ is saying, maybe there's a frame-perfect set of conditions here that could make this happen. Doesn't seem like there *should* be, based on how the components are supposed to work, but things don't always work like they should.
I'll keep an eye out, as well. For now, I guess we can chalk it up to a hyperspace anomaly
(And now, I kind of want to put in a chance for transverse jump to legitimately do that...)