The Meltdown-Spectre patch will not be installed on PCs running the affected anti-virus products unless the vendor in question has added a registry value in the Windows configuration database, to show it's safe to update the system. The bugs were expected since the update rolled out in emergency after flaws in Intel, AMD and ARM chips were discovered. After Microsoft recently confirmed that it is mitigating the Meltdown vulnerability with automatic updates, reports have fallen in that Microsoft's fix for the bug, may be crocking AMD-powered PCs.
This is only affecting some AMD machines, but clearly enough have been hit, because Microsoft has made the decision to halt patching AMD PCs in order to resolve the problem.
The investigation revealed that affected AMD chipsets did not "conform to the documentation previously provided to Microsoft to develop the Windows operating system mitigations to protect against the chipset vulnerabilities known as Spectre and Meltdown".
Which on one hand is great because if you've got an AMD chip and a non-compliant anti-virus package, although it may seem like you're being thrown to the dogs, you're actually avoiding having your machine borked more seriously.