@Topologist nailed it
I use the Discourse-classic theme.
On my Windows tablet using Edge with a maximized window I see exactly what @aurora_maire posted: 5 avatars to the right of (not below) the thread title. And to the left of the view & post count. That seems utterly familiar to me. At least that’s what I see in landscape mode.
If I rotate the tablet to portrait I see only the latest poster’s avatar at the far left, before the title. Likewise if I adjust the browser window width wider or narrower while in landscape mode the avatars change back and forth from 1 on the left to/from 5 on the right as I widen or narrow the window.
The same theme on my phone shows only the latest poster’s avatar at the far left, before the title. This is true in portrait and landscape.