This issue seems to occur due to the client keeping the NetGUID for the class/object, while the server will clean up this NetGUID during travel.
When the client sends the class and its NetGUID as part of the RPC, this results in the following warnings on the server:
LogNetPackageMap: Warning: GetObjectFromNetGUID: Server re-loading object (might have been GC'd). FullNetGUIDPath: [53]/Game/ThirdPerson/Blueprints/PrimaryAssetBP/MyTestPrimaryAssetBlueprint
LogNetPackageMap: Warning: GetObjectFromNetGUID: Attempt to reassign read-only guid. FullNetGUIDPath: [53]/Game/ThirdPerson/Blueprints/PrimaryAssetBP/MyTestPrimaryAssetBlueprint
LogNetPackageMap: Warning: GetObjectFromNetGUID: Attempt to reassign read-only guid. FullNetGUIDPath: [53]/Game/ThirdPerson/Blueprints/PrimaryAssetBP/MyTestPrimaryAssetBlueprint.[55]MyTestPrimaryAssetBlueprint_C
Create an actor class that is marked as a primary asset to be loaded on demand.
On the client, load this asset before sending a server RPC triggering the server to spawn an instance of this class.
Perform a seamless travel.
After travel, send another RPC triggering the server to spawn the actor.
See attached repro project and linked UDN for more info.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-212693 in the post.