G&T,
The conversion of the track into a route seems to be working OK for the bulk of the bold magenta line route, agreed?
The two strange straight lines, the first running east to west, the second west to east, link what I would call ‘spurious points’ that, whilst they appear (to BaseCamp at least) to be a part of the track and therefore need converting and joining together, are not. Hence the straight lines. I see it on some tracks that I import from third parties, like Tourenfahrer magazine.
You can:
A. Try Paul08’s suggestion; let us know if it works, please
B. Try to edit out the erroneous points. Let us know again, please
C. Trace over the route or track, thereby creating a fresh final version. This latter method works well, especially when the track contains some mistakes made by the rider of the original, which I sometimes see on tracks which I have imported from third parties and (naturally enough) mistakes I have made myself