Why does the purple line stop being displayed half way through following a course which has been synced to Fenix 6 sapphire?

Why does the purple line stop being displayed half way through following a course which has been synced to my Fenix 6 sapphire?

I synced a course in connect and transfers to my Fenix 6x. I start a trail run, select a course to follow and the purple line shows me the course on my watch. Then for no discernible reason, the purple line disappears mid course, as if my watch isn’t now following any course. 

this has luckily happened on routes I have been fairly familiar with but has knocked my confidence in using the watch on new courses I have never run before.