Creating a course on Garmin Connect - Elevation data useless

Yep, I’ve got the same issue.  It’s always off.  Has me worried about my first trail race this year so I’m constantly looking for hillier routes to train on.  
However, my daily walks with the dog.  One day I’ll get 7 feet of elevation, the next I’ll get 35.  Same distance.  Same route. 
I feel like it all went wonky a few updates ago.

That's normal, since the total elevation gain is depending on the number of track points. Being the sum of differences between individual neighboring track points, the more track points there are, the bigger the sum will be. It is the same effect as the well know and documented Coastline Paradox. So, if there is significant difference between the number of track points between the Course and the Activity, there will be usually also quite different Total Ascent. And since activities usually have thousands of track points, while courses just tens or hundreds, the difference can be indeed quite significant.

You could do it yourself by increasing the number of course points / way points, but the inconvenient consequence would be that the file would be much bigger, and quite long to load onto the watch. Besides that, many watches have the limit of max 50 waypoints in a course.

However, there are certainly ways to make the values closer to each other, but the problem is that the results will be never identical anyway, since it is not only the number of track points, but also their exact position, and that is in no way predicable. Still, I am sure, it could be done better than it currently is. Feel free to send your suggestions to Garmin through their form at Share Ideas | Garmin 

Great, I've submitted a suggestion. Hopefully Garmin addresses this soon so as to not push people to have to subscribe to another app just for mapping.