Missing activities on FX5+ when trying to navigate an activity (saved on Edge 830) from history

I have a Fenix 5x+ and an Edge 830. Since buying the Edge 830 I use it for all cycling activities. The True-Up feature generally works well and activities saved on the 830 will show up when I go to History > Activities on my 5x+.

Now here is what doesn't seem to work well..

Right now all activities on my 830 are synced with the 5x+ if I try to see them by going to History > Activities.

But..if I am recording an activity on my 5x+ and I go to navigate a course based on a an activity saved, and presumably already synced as described above, on the 830 by doing this:

Long-press Up-Menu button > Navigation > Courses > Activities

...then not all activities (which are already in the 5x+'s history!) are visible from there. I seem to be missing the last three activities.

What am I missing?