When (re)mounting a FS with 'acl' option, this is not showed up in /proc/mounts. As ACL does not work when a FS is mounted without this option, this could be worth to publish this information though /proc/mounts the same way as other mount options.
That's how it's expected to work. The kernel tracks _some_ of the flags in /proc/mounts (specifically, those ones which are the same across all filesystems); others are tracked in user space via /etc/mtab. The mount(8) binary assembles both of these to provide mount information. eg. # mount /dev/hda6 on / type ext3 (rw) # mount -o remount,acl / # mount /dev/hda6 on / type ext3 (rw,acl) # mount -o remount,noacl / # mount /dev/hda6 on / type ext3 (rw,noacl) # That has been the defined behaviour for ages; if you want to see all the information, call mount(8) directly, don't just parse /proc/mounts.