You shouldn’t be using the PBR Master node for this, you should be using the Unlit Master node. Most of the built-in particle shaders are not lit, and while the LWRP does come with lit particle shaders, the default one is not.
To replicate the default particle shader, it should look more like this:
Though, the LWRP unlit particle shader actually has a bug, and the above graph doesn’t exactly replicate the LWRP\Particles\Unlit shader when set to “Transparent, Alpha, Multiply”. You’d need to multiply the final color by the final alpha, which is totally wrong, so don’t do that.
The default particle texture is also kind of terrible since ideally the RGB should be solid white and the blurry blob shape should only exist in the alpha.