I've been using Runna for a few months.
I paused a Garmin Coach plan I had running for a race at the beginning of December, and let Runna take over. When I selected a run workout I was prompted to do the Runna workout.
I now have a Runna plan for another race, and until recently (last week, I'd say), I had Runna daily suggestions when I selected a run. I now get Garmin daily suggestions. However, if I dismiss those and select training>workouts from within the run activity, I can find a load of Runna workouts, including the one for today (and confusingly the one for the previous race with a very similar name). So they sync to the watch, but don't show up as the main suggestion. I may try pausing Garmin daily suggestions...
This is similar to the case reported in an FB Garmin Forum: https://www.facebook.com/groups/garminforum/posts/2668839766809274/