Looking at the client debug logs for the failure, it doesn't record any statfs RPCs at all during the test, and the exports are all reported IDLE. That should definitely not be the case for "lfs df" and "lfs df -i", since the client needs to contact all targets to get the statfs info. Only the "df" statfs can be aggregated, but it shouldn't be cached more than a second. The MDS logs show some activity, but not any of the actual statfs results, because the debug level is too low.
It looks like the test is not running with full debugging enabled as it should for all sanity tests, so I pushed patch 34493 to ensure this is the case, and hopefully we will get some more usable debug logs after that patch lands.
I suspect the last issue was fixed with
LU-12248.