with

Run block with value as the current process, restoring whatever the thread held before - which is not always nothing.

reset clears the slot outright. That is right for a pooled worker that arrived empty, and wrong the moment a task runs on a thread that is already inside a process. An java.util.concurrent.Executor is free to do exactly that: a direct executor always does, and a java.util.concurrent.ThreadPoolExecutor with java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy does once its queue fills - so the behaviour appears under load and not in development.

The submitting thread then comes back from the task holding no process. Nothing throws there. The next AgentProcess.get returns null, typically a blackboard read some distance away, so the symptom is "the blackboard lost my object" and the cause is several frames back.