When a transform used for custom simulation space is deleted the particle system's simulation space is not reverted to behave as if the variable was unassigned.

Repro steps:

Open attached project and scene "Scene
Enter play mdoe to observe custom simulation space
Delete object "GameObject"
Observe "Missing (Transform)" assigned to custom simulation space variable and the offset applied to the particles emitted

Reproducible in 5.6.0b6
Not reproducible in 5.5.x because Vlad says so