In case it offers more insight - I have a simple test map with a call to a newly added blueprint function library, invoked in the level blueprint. It is important to note that this level was saved successfully using a manual save after adding this.
Now, even on subsequent boots of the editor (using the previously saved level), this issue now triggers for me with a 100% reproduction rate whenever an autosave occurs. Manual saves appear to be unaffected prior to this auto save (I can move an actor and trigger a manual save without issue), but is broken after the autosave. Deleting the nodes and recreating solve the problem as suggested - and allows a manual save again. But a subsequent auto save will trigger the issue once more. I’ve had to disable auto save in the meantime.