There are several possible errors that could be made.
Provide some code samples for your pointer management. It will be helpful. (struct field declation in AICharacter, FRunnable method call and maybe that method implementation details)
Or you can try to take closer look into my abandoned (sigh) project https://github.com/RxGamesLtd/USS13:
- FluidSimulationManager - FRunnable
- WorldGrid - actor, that holds and controls that runnable