If you use a Garmin, there are a LOT of bugs associated with course routing and navigation. One way to potentially limit those, is to primarily use the Garmin Connect platform as the "gateway" for what you load on your garmin. So here is a workflow that seems to be a bit better than just grabbing the GPX file off Strava.
1. Make your course wherever you want.
2. Export it as a GPS Tracks file (GPX).
3. Convert it from that export to the same thing (GPS Tracks) via https://www.gpsies.com/convert.do
(yes, I'm not joking about that.. it sanitizes it in a way that allows garmin connect to bring it in)
4. Import the newly converted GPX as an activity.
5. Click the gear and select save as course.
6. Set any pacing info you want, pick a name, and click save.
7. Delete the activity you just brought in (it will be December 31, 2009) by hitting back a few times and then deleting it from the gear menu.
8. Use Garmin Connect to push the course to your Garmin.
Complicated, a little. But... seemingly a bit more reliable about actually working while on the bike.