Repro steps:
1. Open "delme012345.zip" project
2. Load scene "UV1map"
3. Select Main Camera
4. Set Rendering Path to "Legacy Deferred (light prepass)" or "Forward"
5. Observe game view, notice the smooth, round highlight
6. Set Rendering Path to "Deferred" and you'll notice the rough, pixelated highlight
Note: Picture attached

Actual: Rendering Path set to deferred makes the highlight appear rough and pixelated
Expected: Rendering Path set to deferred should make the highlight appear smooth

Reproduced on: 5.5.0b1, 5.4.0p3, 5.3.6p3, 5.2.4f1, 5.1.4f1

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Issue won't fix because the artifact in the specular highlight is a result of the deferred gbuffer quantization, which is an inherent drawback of deferred rendering.
Because it's only visible in smooth, low-curvature areas, it's usually best just to workaround it in content:

1) use flat geometry, or high curvature, but not low curvature;
2) use a non-flat normal map -- some slight wobbles in the normal, or a micro-detail, will obscure the quantization;
3) create a new shader that adds a bit of screenspace noise to the normal to dither it and break up the facets. This gives a very slightly sparkly appearance, but without faceting. I attached an example shader to the bug if you are interested.