Hi,
Looks like you found a bug! 'Ack Time' and 'Clear Time' columns when not available are supposed to be blank (as they're in the text report), but apparently it's not in the HTML report.
A workaround for now would be to create your own HTML report using the Alarm & Event Query VI. You will have to index the array, and then unbundle the cluster and extract the values while embedding them in HTML tags. This is easy to do but can be quite time consuming.
Also, this approach will allow you to strip the trace URL and get just the tagname.
Hope this helps.
Khalid