I was one of those plagued by the "stopping at 99%" GUI error until the recent build, but now I've got a different problem: when I click on the Details button, either in the pop-up notification or in the notifications list, the completed scan starts over. This has happened with both Quick Scan and Full Scan. I am only able to view the details by going into Protection > Antivirus - Scan History and selecting the relevant scan report.
So far, I have tried repairing the installation, uninstalling and reinstalling, and then repairing THAT installation; the problem persists. (This is on Windows 10.) Is this a known bug, or am I just lucky?