"Unread" tab is not listing all thread with unread messages

When I go to “unread”, some threads are not listed, even though I know they have unread messages in them. Just compare the current “latest” view I get…

… with the “unread” view:

As you can see, there are 4 threads with unread messages immediately visible under “latest”, but one of them is missing from “unread”. “unread” is how I keep up with conversations so this incurs a high risk of me just entirely missing that there is something I should reply to.

I am seeing this both on https://internals.rust-lang.org and https://discourse.llvm.org/, which are both DNS aliases for something under hosted-by-discourse.com so I assume they use the latest version.