Panic because sorting function doesn't implement total order

[poljar]

We seem to have a panic in the codebase because one of the functions we pass to sort_by() doesn't implement a total order.

A sentry report was sent to use with the following message:

user-provided comparison function does not correctly implement a total order

Sadly the stack trace doesn't really tell us much which function in our codebase called the sorting function.

I attempted to guess which one it could have been in https://github.com/matrix-org/matrix-rust-sdk/pull/5782 but the recency sorter seems to be fine.