This is where I've had the GPS antenna for the entire time, including the previous K2 system:
In case you don't speak Vstrom, that's right at the back end of the seat, at the front edge of the "carrier" area. I have a big aluminum plate there where the top case attaches.
I can't say whether I had consistently reliable GPS logging with the old setup (DVR in front, GPS antenna as shown here) because I had the DVR constantly being shut down, whenever it was warm out. As we now know, that was due to a fault in the previous power converter design. I was preoccupied by that problem; GPS logging was secondary.
Now that I seem to have the power issue sorted, I've been noticing the GPS problem.
Moving GPS antenna to top case this evening, we'll see whether it makes a diff. The EE in me says it shouldn't, since the plastic top case (usually containing nothing more than a helmet bag and windshield towel) should be transparent to GPS signals. But, he also says that RF stuff is weird and unpredictable.
FWIW, here is at least one other person having a similar issue to mine:
Why is it some of the recordings from the K2 do not gps data?
www.innovv.com
You'll note his antenna placement is utterly irreproachable.