thank you Walter for your insight, much appreciated.
I didn’t adopt your solution, but would consider it for next time.
For anyone browsing this thread, I was able to render the particles as a separate animation, then bring that back in, and then used a mask effect then worked.
I believe the real reason why particles and its attendant mask, both sourcing within the same layer, cannot be done – may be done to technical insufficiency of the app itself. Perhaps a later version of AE can do this, but I can’t (yet) confirm this.
Warm regards,
Chris