NOT fixed as of 4.21.1. I don’t know if it was actually fixed in 4.21 and it was re-introduced in 4.21.1, but it is definitely still recurring.
Specifically, I have the following declared in a class derived from UActorComponent:
UPROPERTY(EditDefaultsOnly, Category = "Setup")
TSubclassOf<AProjectile> ProjectileBlueprint;
Projectile Blueprint is reset to None every time the editor is re-opened or Hot Reloaded.