The problem has recurred on my device, and I've managed to gather a sysdiagnose. By analyzing the logs, I think I've been able to track down one of the issues. This is the error I am seeing:
Failed to notify (null) that appLimitThreshold's threshold was reached: Error Domain=PlugInKit Code=4 "RBSLaunchRequest error trying to launch plugin com.jomo.Jomo.JomoDeviceActivityMonitor(8104FF60-5C49-45BD-8AFB-97BE88488134): Error Domain=RBSRequestErrorDomain Code=5 "Launch failed." UserInfo={NSLocalizedFailureReason=Launch failed., NSUnderlyingError=0x5dd8273e0 {Error Domain=NSPOSIXErrorDomain Code=111 "Unknown error: 111" UserInfo={NSLocalizedDescription=Launchd job spawn failed}}}" UserInfo={NSLocalizedDescription=RBSLaunchRequest error trying to launch plugin com.jomo.Jomo.JomoDeviceActivityMonitor(8104FF60-5C49-45BD-8AFB-97BE88488134): Error Domain=RBSRequestErrorDomain Code=5 "Launch failed." UserInfo={NSLocalizedFailureReason=Launch failed., NSUnderlyingError=0x5dd8273e0 {Error Domain=NSPOSIXErrorDomain Code=111 "Unknown error: 111" UserInfo={NSLocalizedDescription=Launchd job spawn failed}}}}
Basically, it seems the system is failing to launch the device activity monitor extension. As a result, the eventDidReachThreshold won't be triggered. This issue may persist for several minutes or hours once it begins.
By checking a previous error, it appears the system is failing to launch the extension because it's looking at an invalid path:
Service could not initialize: access(/private/var/containers/Bundle/Application/C6598B47-8977-447C-870B-4D21BDE8ACF9/Jomo.app/PlugIns/JomoDeviceActivityMonitor.appex/JomoDeviceActivityMonitor, X_OK) failed with errno 2 - No such file or directory, error 0x6f - Invalid or missing Program/ProgramArguments
I've filed a new bug report (FB13556935), which includes the sysdiagnose and precise timestamps of when the issue was reproduced. I think this bug is CRITICAL for all apps relying on the Device Activity framework. It likely explains a range of issues reported since the framework's release. Specifically, due to this bug, end users may encounter problems such as apps not unblocking at the end of a schedule, apps not blocking at the start of a schedule, and time limits not being updated, among others.