For me, NAV did not work from the takeoff. I've never turned heading mode. After takeoff it order FD to turn right (I was following FD, and noticed early turn right, and then constant turn). And whenever I tried to switch back to NAV, it was always, constantly trying to do right turn.
I've deprted EDDH, route created using simbrief, and uploaded route to FMC with uplink as well.
my route:
EDDH/33 N0473F270 AMLU1G AMLUH M852 POVEL Y800 DESAR DCT UPLEG UPLE3M EDDP/26L