System Information
Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GTX 980/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 425.31

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-12-04 11:32, hash: rBf1aa4d18d49d
Worked: Blender 2.79b

Short description of error
Skin modifier is unable to display edit cage (modifier display mode #4) for loose edges.

This happens if a Mask modifier hides vertices to build these loose edges.
As you can see, faces are still displayed correctly.

Exact steps for others to reproduce the error

1. Create a Mesh (subdivide it 1 time or not).
2. Give it a Mask modifier and activate "On Cage" Edit mode.
3. Give it a new Vertex Group and mask geometry to create loose edges.
4. Give it a Skin modifier with visible origin and activate "Display in Edit mode".
5. Toggle X-Ray to see the edges.

Issue:
Loose edges created by the Mask modifier wont appear, but are still clickable/movable.

In the file I have added a monkey mesh to show where this behaviour can cause problems while editing.