The project given by the user has AnimNotifies set up on the ThirdPersonIdle pose.
This notify will spawn a particle system when triggered, and the event has a 0.5 Trigger Weight Threshold.
Regardless of whether or not Sub Anim Instances ("Use Subgraphs") is enabled, the notify should only occur when the blend weight is less that 0.5 (e.g. ThirdPersonIdle's blend is greater than 0.5)
When Sub Anim Instances are disabled, the notify is only triggered when the blend weight is less than 0.5
When Sub Anim Instances are enabled, the notify always triggers.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-47866 in the post.