Probably something with federation running behind.

Like, you’re instance A, one of those accounts is on instance B, and the ones you can’t see are from instance C.

So A has an issue with C, but B doesn’t.

So A (you) can see comments from A and B. But B can see comments from A,B, and C.

A doesn’t see the C’s, but knows B has replied.

It’s also possible your app is the issue if not using web client.