Same issue here.
In my case I have a niagara FX in the sequencer that has an exposed user parameter for a spline component.
The spline component ref is set via an actor that has it in the map.
If I convert the niagara FX to spawnable it complains it needs to find the ref and when it tries it can’t find it so I can’t save. I need it to be spawnable though for replication reasons.