This bug bit me too, because I have scripts that automatically mount snapshots and do some processing for a SQL process I've created. Once the December backups were gone, it started working again. Fortunately, I'd fixed the problem by then.
"Sort-Object Created" will not work. "Created" is not the actual field name for the date the snapshot was created, even though it's column display name is "Created".
I work around the sorting problem by doing this...
get-nasnapshot -Name volname | Where { $_.Name -like "sqlsnap*" } | Sort-Object AccessTime,Name -descending | Select -Last 1
This will get me any snapshot named "sqlsnap*", so I'm filtering out the automatic snapshots for my snapmirror process, etc.. Then I sort the object from most recent to least recent by the date the snapshot was created and the name of the snapshot. The name is just in there as a safety measure, I guess. There's little chance that two snapshots could have the same exact creation time.