This bug may already be fixed. I am running 3.8.2, not a later beta. I notice when I have an "out and back" track, that is the track goes in both directions in some locations (eg on same road), that the navigation mode doesn't work correctly. Here is whole track in Training Center - out & back + loop:

I used BRouter without Computing Instructions, then I converted 14 points to navigation waypoints. Only the first was ever displayed in navigation mode (maybe because the track didn't overlap yet). After moving past the first point, the purple track was shown thin in both directions, and Locus displays question mark for distance to next waypoint (note arrows in both directions below):

Once I got to the loop I cancelled the navigation, then restarted, and the remaining navigation worked OK.

Also when I was editing the track, converting points to navigation waypoints, once I got to point #682 (finish the loop, and back along same road) tapping the right button would start moving the cursor back around the loop again instead of moving towards the end, so I had no way to convert anymore points.
I've attached the TCX too.