Using actors locally on a single machine is one of the worst ways to structure a program. The pattern foregoes specific deterministic testability.

Specific = Under the programmer's full control, not changed by updating dependencies.

Deterministic = Always yields the same output for the same input.

Testability = Ability to test the program using automated testing.

If the intent of actors is to exploit parallelism, then why not use deterministic data parallelism libraries such as rayon? Otherwise abstract nondeterministic concurrency properly so it becomes deterministic.
Specific deterministic testability is one of those properties that allows us to easily debug and test a program. As long as we're on the same machine we should seek to protect this property of our code.