I noticed this problem on my flight from ORD-HKG. We took to polar route going "east" the same route your map initially showed. Then I also noticed that it suddenly changed. I know it was wrong because I was recording the GPS route on my phone so I know how we flew. (Plus I later looked at it on flightaware, luckily it caught enough of the route)

I'm pretty sure it is a software bug. I wouldn't put too much trust into its accuracy.
My phone may not be that precise, but I trust it more than the screen.