Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 440.97
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
Short description of error
When unwrapping, if you pin some faces in an island, then the other islands will not be in the same scale anymore, they will fill the whole map instead of remaining in the same scale.
Exact steps for others to reproduce the error
-before unwrapping any object, add some seams around a few faces so that this area will have its own small island in the UV map
-unwrap the object. You should now have a main island, and a small island in the UV map
-pin a few faces of the main island in the UV map
-unwrap the object again. Now the small island will fill the whole map or be way too large, instead of remaining in the correct scale.
I tried in Blender 2.81, and the problem is still there.