That's a really hard question to answer without seeing the report and what options where selected when creating it. Currently there are no support cases in with Tier 3 on trend reports and in general they work pretty well. I suggest opening a support case so we can take a look at the report.
I had a some extra time today so I created a trend report based off of your exampe below. I scanned a system for 10 days that only contained one high vulnerability.
I recieved a Foundscore of 30 for each data point over each of the 10 days.
I've attached the report with its configuration settings to this post for you to take a look at. It sounds like you might have something configured incorrectly on your system. Please open a support case and I'm sure we can get this worked out for you.
Have a great day
Thanks for your reply.
I have tested trend asset reporting on a new vm image, fs v6.8, and the reports are ok.
I will open a case and hope to figure out what is happening.