AFAIK UE4 doesnt handle sound virtualization and instead when a sound it inaudible (volume is at 0) it shuts the sound off instead of virtualizing it and keeping the sound going linearly in memory. So whenever you leave the attenuation the sound is stopping and when you reenter the radius it is indeed restarting.
Thats what is happening. How to make it work in UE4 I dont know, outside of using Wwise which does have sound virtualization. You could potentially try using Attenuation Inverse with a crazy large falloff radius so that the sound will be playing even if you move far away but is only heard when your pretty close. Using that and some AudioVolumes for occlusion might be a quick fix.