I’ve found the problem but haven’t made sense of it yet.
The problem BP node was Push Sound Mix Modifier but the pushing alone wasn’t the issue it was the file it was pushing. I instead made a new “Sound Mix” file and used that instead and the crash went away.
The original sound mix file was made in 4.20.2 and I can only guess moving from it to 4.20.3 caused the issue or something else went wrong?
If someone can advise how I can drop down to 4.20.2 to reproduce great if not I’m not sure what else to say other than if someone else runs into this, see if removing “Push Sound Mix Modifier” helps and if it does trying making a new Sound Mix file and use that instead.