Cannot remount an APFS filesystem as read-write

I have a local APFS filesystem (sparsebundle disk image) mounted read-only:

/dev/disk5s1 on /opt/homebrew (apfs, local, nodev, nosuid, read-only, journaled, noowners, nobrowse)

There are no open files as reported by

$ sudo lsof +f -- /opt/homebrew

However, an attempt to remount via the -u and -w options fails with the following error:

$ sudo mount -u -w /opt/homebrew
mount_apfs: volume could not be mounted: Permission denied
mount: /opt/homebrew failed with 66

Is there a configuration option that I'm missing?