Intel's attempts to mitigate side-channel vulnerabilities in practically every processor released in the past decade has not come without consequences. There was the whole random reboot issue that affected Haswell and Broadwell CPUs (it was later resolved), and now there are reports of a recent Windows 10 update is wreaking havoc on some overclocked settings.
The issue appears to be specific to Broadwell-E processors, and relates to Microsoft's KB4100347 update. That patch deals with Spectre Variant 2, and is based on Intel's microcode update for Broadwell-E and various Skylake products.
"I just got around to installing a handful of updates from the past week on my computer, and upon rebooting I realized that the overclock on my Core i7-6800K was no longer applying. I figured it might be because of the microcode updates, and sure enough, that appears to be exactly the case," a user on Reddit wrote.
That is not the only forum thread on the matter. Over at Asus, a user complained of the same thing.
"I wanted to warn you guys for Windows 10 update KB4100347 ( Intel CPU Microcode update) this one kills UEFI Bios overclock. Before the update my system was running 4.2GHz on all cores but after this update the CPU runs default 3.7GHz all cores with 1*Core @ 4GHz, so yes default spec," the user wrote.
If you're affected by this, there are a few things you can do. One is to shake an angry fist at the CPU gods and swear off overclocking, though they are not the ones to blame. Another option is to uninstall that particular update.
Fortunately, it's reported that KB4346084 resolves the issue. Several users report that uninstalling KB4100347 and installing KB4346084 restores the overclocked settings, though it's not clear if the former update actually needs uninstalled—the latter one is supposed to override it.