Think I am been a bit dim here.
No - what you describe is a bit puzzling.
I've tried to replicate it (I don't use Tracks much) on my Legend HCx and did get something similar though not exactly the same.
In my case, it was a matter of selecting the Track more precisely (so, contrary to the advice I offered in a previous message) - ie by zooming in on the map until I was definitely clicking on the Track and not on some nearby map feature. I was then prompted 'Track or Route' and I selected Track.
It also helps if you are located on the Track when you do this - if you are some distance off it (as I was when I tried it just now), the GPS will then insert a 'Go To' to get to the nearest point on the Track - which kinda confuses the issue.
The prompt for 'fastest or shortest' is one I've never seen, but it can only be a Routing prompt so is not what you want to see. Maybe it was the 'Go To' kicking in.
Maybe suggesting TracBack wasn't such a good idea in the 1st place ...