I can confirm this works, I’ve turned off “Auto Disable” on both of the submixes and now the Niagara Systems are behaving as expected
When hovering over the parameter it reads:
Auto-manage enabling and disabling the submix as a CPU optimization. It will be disabled if the submix and all child submixes are silent. It will re-enable if a sound is sent to the submix or a child submix is audible.
It makes sense that they get disabled if not used, but it looks like they are not getting re-enabled…