1. Go to the website home page and count your online friends in the wing.
  2. Scroll down in the wing till the first time the scrollbar changes size (loading more entries)
  3. Scroll back up to the top.
  4. Observe that there are roughly double the number of friends online than when first observed.
There doesn't seem to be any pattern as to which friends don't show up other than that it doesn't affect friends where their current instance is visible.
It's been an issue since the "online on other platforms" update.