System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1050 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.19

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-08-05 22:29, hash: rB675700d94892

Short description of error

UV Island won't get unwrapped if only one vertex in the island is pinned.

Exact steps for others to reproduce the error

1. Open attached model
2. In UV Editing view, select one vertex and pin

3. Select all and press unwrap. Nothing happens

4. Select another vertex and pin

5. Select all and press unwrap. Now it works