The out of the box alert doesn't distinguish between local and shared shared storage. The scope of the search is
host.vol.freeSpacePercentage:[* TO 20]
If you click on the alert name, you can see the list of volumes that triggered the alert in the 'Least free space' column.
If you want to restrict the search (i.e., evaluate the alert) for certain volumes only, you can edit the Alert and change the search to something like
host.vol.freeSpacePercentage:[* TO 20] host.vol.mountPoint:*LUN*
to restrict to volumes containing the word LUN
this is caused by the fact that the application started collecting more data about non-local storage since the previous version. Unfortunately, there is currently no reliable information under host.vol. that would indicate whether the volume is local or not, so I'm afraid there are only heuristic solutions similar to the one proposed by HolyGuacamole. I'm curious about why is alerting on local storage separately important to you?
Anyway, the VMan team is now aware of the issue and will consider fixing it in a future release.