The straight line means GPS stopped. You could have hit pause or somehow lost signal. For example, I was tracking our long walk (as tourists) in Guanajuato, Mexico last week and kept the walk app running all day. We were walking all over town and through all of those narrow lanes for many miles. Once I jumped in a cab and hit pause. The cab went about a mile and I got out and resumed the activity. The track later showed a straight line like that.
Datafields in the activity are fine, but if you exit the activity, don't open 3rd party app/widgets. Its being worked on by Garmin, hopefully fixed in the upcoming beta but I don't have any information besides its known to Garmin. You will also feel the watch vibrating signaling the recording has stopped.
