I think that whatever the emitter is, you won’t see the particles in a screenshot by using CreateScreenshotUsingRenderTarget.
After some source digging, it seems RenderTargetTexture deals only with meshes: the _renderList property used internally to render objects in the texture is an Array<AbstractMesh>, and particle systems are not AbstrachMesh: the list of objects is retrieved through a call to Scene.getActiveMeshes() whereas particle systems are found in Scene.particleSystems[].
Don’t know if it’s a bug or if it is expected… Looks like a bug to me, however. Maybe @Deltakosh will have some insight about this?
You should use CreateScreenshot instead, which will work as expected.