Once you've fixed an issue, it will display in your account until another scheduled scan is run or you manually start a new scan. If the issue is fixed, it will no longer show in your dashboard after the new scan.
If you notice recurring false positives in your diagnostics results (e.g. slow loading pages that are PDFs, links incorrectly appearing as broken), simply check the box next to the false positive and click the Hide Checked button at the bottom of the screen.
While scans will keep detecting the issue, it will no longer appear in your dashboard. You can always see hidden issues by clicking the View Hidden icon in the top right corner within any Diagnostics page.