We’re still looking at what’s causing the issue, but it looks like it may be something with the latest Zwift update.
We did find a potential workaround in the meantime:
Just dragging the TR workout to a different day should cause the TrainerRoad folder to show up in the Zwift Custom workouts section. However, if we then restart the Zwift app, the TR workout disappears. (once again, this is all iOS, desktop continues to behave as normal).