Oops!
It returns them as UActorComponents thus a simple cast should suffice:
I guess you could use
TArray<UActorComponent*> Comps = GetComponentsByClass(UBrushDynamicComponent::StaticClass());
and then cast it to your UBrushDynamicComponent each time you use it.
For the simple GetComponents function did you try omitting the TSubclasOf part? A subclass is also the original class and should be returned eitherway