It is the correct behaviour.

You use the `-f` flag, which mean:

> `-f`, `--fake`:
> Causes everything to be done except for the actual system call; if it's not obvious, this **``fakes''** mounting the filesystem. This option is useful in conjunction with the `-v` flag to determine what the mount command is trying to do. It can also be used to add entries for devices that were mounted earlier with the `-n` option. The `-f` option checks for an existing record in `/etc/mtab` and fails when the record already exists (with a regular non-
 fake mount, this check is done by the kernel).

See also https://unix.stackexchange.com/questions/195010/remount-a-busy-disk-to-read-only-mode.