I noticed that the md_stats proc file was empty, although there was activity on the filesystem and exports/*/stats contained several stats and were being updated.

I unmounted and remounted the fs, no change.

I rebooted, and md_stats contained usual operation stats data.

Then remounted the mdt, and md_stats stayed empty again.

Also, fs/lustre/mdt/*/stats is empty, and remained empty after a reboot and remounts.

My normal procedure for creating a lustre target does do a remount after the initial registration step, so it's possible this only occurs after remounting the mdt.