As proactivesvcs commented under the other article, I think it was just insufficient down-scrolling. The Mastodon web UI on a phone can make for a very long page.
Edit: wait a sec... something IS weird here
I now have the same URL showing in two different tabs, one of which shows the follow-up and one which does not. It's a UI matter of which part of the tree of follow-ups is being displayed, and it's not encoded in the URL.
(Still editing) OK, if you are in the state where it does not show the entire thread down to the finale, click on the text of the first follow-up by the OP ("The toothbrush thing has gone viral despite it being total bollocks") and the rest of the thread is revealed.
Except it's not. That's another part of the tree with the same screenshot in reply to a different subthread.
All this goes to show that I don't understand how the web UI of Mastodon works, and I'm not alone. "When you're in a hole... stop digging"
First reply is from the OP, and the visual "thread" (vertical line) on the left side is broken. Click on that reply ("The toothbrush thing has gone viral despite it being total bollocks.").
Now again scroll down through posts by the OP until the "thread" on the left side breaks. Click on that last reply ("Fortinet also declined to comment to me.").
Finally, the target post appears as a reply to that ("Fortigate have issued me a statement.")
Navigational issue. One might ask what is the difference between the follow-ups that show up automatically and the ones that don't show up until you navigate. I don't know.
https://news.ycombinator.com/item?id=39296438