When that happens to me it's because I've blocked whoever made the comment. I've never experienced 13 people I blocked all commenting on the same post though so I'm not sure if that's what's happening here... Do you block a lot of people?
If you haven't blocked anyone, any communities or instances then check you have both English and Undefined selected in language settings (along with other languages you want to see posts/comments in).
I wonder if this is related to sync having the same issue. I frequently find threads with 5-10 comments but nothing shows up. I don't have anybody blocked whatsoever and the issue happens on posts from various instances, even my own. Maybe this could be a bug that affects both apps. When I load these threads in browser it looks fine.
All 13 of them? I don't block lemmy.world, and it's unlikely there would be that many from hexbear, and I've only individually blocked a couple of people.
Voyager often has strange sync issues. I find that attempting to reload the post (just pull down while viewing comments) will often work to load the comments. If that doesn't work, I find that reloading the entire feed will generally fix the issue
I thought lemm.ee might be lagging behind as it shows fine in my voyager, but the comments display just fine on lemm.ee. Have you tried refreshing? Sometimes I have this happen and just a refresh will load them.