I wasn't sure whether to start a new forum topic, a new help topic, or (finally decided) it was maybe a 3.8 beta issue so added to this topic.
I created a track, converted 14 track points to navigation points, then navigated the track. No problems so far. In fact with the new 15s display on time it was the best hands free navigation experience to date.
I then copied this track to test the return navigation (also partly to workaround the "loop" bug). I left Compute Instructions unticked to preserve existing course points. I then navigated the track. I initially followed a different physical route (kept recalculate unticked) so first 4 navigation points were not announced. However of the remaining 10 navigation points only 3 were announced. If I manually turned on the screen I could see a blue dot icon for each of the other 7 points, but didn't activate the display or generate an announcement. Why would 3 points work correctly, and not the other 7? I exported both tracks to TCX & have attached. I also notice that none of the track or course points have an actual timestamp - seems to instead be #seconds since 1970-01-01T00:00:00Z - for example last track point is 1970-01-09T11:09:16Z
I can possibly answer my own question - it appears when a track is copied (tracks > More > Create copy with reversal), any straight or U-turn navigation waypoint is not copied. All other 19 navigation waypoint types are copied (!). As a test I created a track with all 21 navigation waypoint types. I copied the track with reversal. And the 2 types mentioned don't appear as a waypoint. Pretty simple bug. If I copied the track without reversal all 21 navigation waypoint types are copied.