Starting on Saturday, Sept. 9 (as far as I can tell) the security_DeviceJail_Lockdown, security_DeviceJail_Detach and security_DeviceJail_AllowDeny tests all started consistently failing. You can see this at the bottom of the wmatrix page if you filter for the 'security' tests and then look for the failing tests:

https://wmatrix.googleplex.com/security

You can also see the failure in the toolchain waterfall builders, starting on Sept. 9 (the tests passed in those builders on Sept. 8).