UAIPerceptionSystem not ticking!

Hi
When investigating a PerceptionComponent->GetCurrentlyPerceivedActors call not working, I tracked it down to the UAIPerceptionSystem::Tick not being called.
And , indeed, when stepping through FTickableGameObject::TickObjects (after building the engine in debug mode), the client where the problem happens has 16 items in the Statics.TickableObjects array while the other clients where everything work have 18 items in there.

Weirdly, the system ticks correctly in my main menu and other levels, but it seems that travelling to a specific map triggers some code that removes two objects from the tickable objects.

Does anyone know what could be the reason for this? I thought these systems were singleton whose lifetime was the same as the process itself.

TIA.