Actor Soft References which have been set in the class defaults (not set per instance) don't seem to get the PIE path fixup when they're private.
Toggling the property as private and public shows different paths for the same property:
Users can get around the bug by making their properties public.
Create an actor blueprint, BP_Actor.
In it, create a property for an Actor Soft Reference.
In an empty World Partitioned map, put down a Cube Actor and the BP_Actor.
In the BP_Actor class defaults, set the Actor Soft Reference property to the Cube actor in the level by using the Picker.
In BeginPlay, print out whether the soft reference is valid or not.
Now enter PIE. When the property is Public (Eye Open icon) it will resolve correctly. If it is private, it will not.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-173196 in the post.