AI characters with Navigation Invokers that are spawned during Play do not generate a nav mesh around them if Runtime Generation is set to Dynamic Modifiers Only

With Generate Navigation Only Around Navigation Invokers enabled and Runtime Generation set to Dynamic Modifiers Only, AI characters with Navigation Invokers do not generate a nav mesh around them if spawned during PIE.

Reproduced in 4.9.1 binary and Main (CL 2695010)

1. Open the attached project
2. Simulate
3. Stop PIE
4. Change Project Settings > Navigation Mesh > Runtime Generation from Dynamic to Dynamic Modifiers Only
5. Simulate

Result:
With Dynamic Runtime Generation, the AI character generates a nav mesh (it uses a Navigation Invoker component), but with Runtime Generation set to Dynamic Modifiers Only, no nav mesh is created and the character cannot move