Azure Durable Functions suspends for hours

I have an Azure durable function that has several activities and then schedules a few tens of sub-orchestrators. It seems to me, that after some time, the function kind of goes to sleep, and doesn't pick up new tasks, finish long-running sub-orchestrations it started originally unless I manually suspend and immediately resume the execution of the orchestrator/sub-orchestrator. some sub-orchestrators have been scheduled for more than 2 hours, even though there are plenty of free instances on the app service plan.

Examples of sleeping for hours between executions:

And idle executions until manually suspending/resuming:

For reference, here's the host.json file:

{
  "version": "2.0",
  "logging": {
    "logLevel": {
      "default": "Information"
    },
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      }
    },
    "fileLoggingMode": "always"
  },
  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[4.*, 5.0.0)"
  },
  "functionTimeout": "02:00:00",
  "extensions": {
    "durableTask": {
      "hubName": "AIHub",
      "maxConcurrentActivityFunctions": 6,
      "maxConcurrentOrchestratorFunctions": 12,
      "extendedSessionsEnabled": true,
      "useGracefulShutdown": true,
      "useTablePartitionManagement": true
    }
  },
  "concurrency": {
    "dynamicConcurrencyEnabled": true
  }
}