Presumably to avoid issues from dereferencing a bind-mounted /proc/self symlink that points somewhere incorrect, the proc_self_*() functions instead open the /proc/{getpid()} directory. However, the /proc/{pid} directories refer to PIDs relative to the /proc mount's PID namespace. Thus, using getpid() can result in the wrong directory if the calling process is in a different PID namespace. For instance, this program prints the contents of /proc/1/status, which is actually the status file of the global PID 1 (systemd on my system):

// rustix = { version = "=0.38.14", features = ["procfs", "thread"] }
use rustix::thread::UnshareFlags;
use std::{fs::File, io, os::unix::process::CommandExt, process::Command};

fn main() -> io::Result<()> {
    rustix::thread::unshare(UnshareFlags::NEWUSER | UnshareFlags::NEWPID)?;
    let mut command = Command::new("");
    unsafe {
        command.pre_exec(|| {
            let mut status = File::from(rustix::procfs::proc_self_status()?);
            io::copy(&mut status, &mut io::stdout())?;
            Err(io::ErrorKind::Other.into())
        });
    }
    let _ = command.spawn();
    Ok(())
}

One solution would be to parse the integer from the /proc/self symlink, then open /proc/{integer} as before. As noted in pid_namespaces(7), it will always point to the current process in the /proc mount's PID namespace (assuming it hasn't been bind-mounted):

Calling readlink(2) on the path /proc/self yields the process ID of the caller in the PID namespace of the procfs mount (i.e., the PID namespace of the process that mounted the procfs). This can be useful for introspection purposes, when a process wants to discover its PID in other namespaces.