I have a similar possibly related situation. I updated from v7 to v8 a couple of days ago and as soon as I did a 'Guest Disk Space' alarm that was triggered in the "Warning" state prior to update resolved itself showing "All alarm metrics are back to normal" in the history. I did nothing to any guest disks on this VM and the error condition still exists but the alarm will not trigger again. On the alarm history it shows the previous warning state with the detail "Amount of free disk space "H:\" (10%) is below a defined threshold (10%). Current free disk space is 19.25 GB." The alarm settings are the defaults, Warning below 10%, Error below 5%, Delay 5 minutes, Assignment Virtual Infrastructure, Action send email to default, and no suppression. There are also no exclusions on that alarm.
One thing that caught my eye is that on the "Guest Disk Space" alarm details in the 'Knowledge' section is says "Guest OS volume is low on available guest disk space". Does this mean that this predefined alarm is only going to monitor the OS volume meaning the system partition, and if so why did that change? I don't know what that 'Knowledge" section said in v7, but as I pointed out from my v7 experience this same alarm was triggered on the guest H drive which is not where the OS is installed in that guest.