For everyone who’s still struggling with this problem, I found the solution.
The issue was with the renderer material. In Mixed Reality (and likely AR too), Additive blend mode doesn’t work well when rendering against the real-world background — the particles become invisible unless there’s a virtual object behind them.
To fix it:
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Open your Niagara Emitter.
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Go to the Renderer section (e.g., Sprite Renderer, Ribbon Renderer, etc.).
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Find the Material being used and open it.
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In the Material Editor, change the Blend Mode from Additive to Translucent or Opaque (whichever works best for your use case).
Note: It’s best to make a copy of the original material into your own content folder, make the change there, and then assign that new material in the renderer. This ensures your setup stays intact if you move the project to another PC.
After this change, the Niagara particles showed up correctly over the real-world background in MR for me.
Hope this helps others facing the same issue.