System Information
Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: Quadro P2000/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.02

Blender Version
Broken: version: 2.81 (sub 1), branch: master, commit date: 2019-08-05 18:52, hash: rBcdeda1fa6cea
Worked: (optional)

Short description of error

Using "0.02" margin for "Island Margin" in "Smart UV Project" menu, you can see the different width around each islands.

Exact steps for others to reproduce the error

This is the result of Smart UV Project by 2.81.

And old 2.78c's result is below.

Both results were using same parameters;Angle Limit 66.00, Island Margin 0.02, Area Weight 0.0.

I attached the .blend.
In this case, all faces are separated by all edges, so all faces should be arranged with same margin around them as the result of 2.78c.

Even if the algorithm has been changed, some Islands doesn't have correct margin.

Thanks.