Operating system: Windows 10
Graphics card: GTX 1060
Broken: (example: 2.80, edbf15d3c044, master, 2018-11-28, as found on the splash screen)
Worked: (newest version of Blender that worked as expected)
When unwrapping polygons without seams and then using "Seams from Islands" feature it erases all the other existing seams
mark seams manually, then select any random set of polygons and make seams from islands in the UV menu, manually made seams will disappear
Based on the default startup or an attached .blend file (as simple as possible).