EDIT: I checked my track more thoroughly and found out that the GPS went of the course after about 4,5 km (after good 40 minutes of run).
Actually the signal sign was all bars, but my mileage did not change. I've only noticed that after 25 minutes when I checked my "Climb" value. I saw the mileage and then after 2 minutes it did not change. That got my attention, of course I continued to run, but I tried to dabble with my watch.
Got it to "distance" screen, then went to breadcrumbs... That was all messed up, the triangle (my pos) was just rolling around and not moving. I zoomed out, zoomed in, it did not change position. Then after a while it started again, and the distance automatically jumped from 13.14 km to 14.65 km. And after that it looked like it's working for a couple km then stopped measuring distance again, till the end of the run. The funny thing is all other screens seemed to work just fine. Elevation was right on spot, HR also, time rolled as normal.
After I got home I connected my watch to PC and got a really weird track of my trail. First part, when everything worked fine was spot on, then I got transferred in a straight line to the west and finished somewhat 4 km from my actual finish. All in all my distance data was wrong, so was my pace (logically) and my position, but the elevation and heart rate were spot on.
I hope it's just a fluke on account of the new firmware cause I'd really like my watch to be fine. I'm having 4 kind of big track & trail races come end of October and beginning of November, so I'd like to be confident this does not happen when I'll need the GPS data form my watch.