Unexpected navigation alerts

First time I have used my newish Garmin Explore. I used RideWithGPS to plan a 66km route. This included out and back legs that traveled along the same bike path for the first and last 16km - there are few turns or intersections, mainly long straight stretches. I went for the ride, and on the outward leg I found the device giving many audible alerts (about every 20-30 seconds where I would expect something maybe every 3-5minutes.

Riding with only the map showing I found that:-

  1. the device sounded an alert 30metres before each navigation arrow for the outward leg and again just as the location icon "Δ" got to the navigation arrow. OK, that's what I'd expect;
  2. the device was also beeping 30metres before encountering the navigation arrows showing for the return leg (i.e. pointing back the way I had just come), and again just before those arrows were encountered. Now that was unexpected and very annoying.

On the other hand, when actually on the return leg, beeps were as expected, and did not beep indicating arrows for the outbound leg.

Is this some setting I have to change? a fault, or undocumented feature?. Can I fix it?  Should the arrows for the return leg show up on the outward leg of the trip?