It looks like a long discussion here [1] broke some code responsible for displaying the comment collapsing buttons. I have visited other threads and can see them working but this specific link seems to be broken. The bottom of the page is also missing and that’s probably why the collapsing buttons are nowhere to be seen.
[1] https://news.ycombinator.com/item?id=23803539
👤 mellosouls Accepted Answer ✓
I'm having no problem with Chrome, Edge or Firefox on Windows 10 on that thread - it might help potential investigators if you list your exact platform.
Also try different browsers and with extensions turned off, and after clearing cache etc.