Operating system: Ubuntu 18.04
Graphics card: nvidia gt650m
Broken: version: 2.80 (sub 40), branch: blender2.7, commit date: 2019-01-08 23:33, hash: 3c3d80ea22af, type: Release
Extruded vertex from subdivided mesh (with shrinkwrap modifier + edit cage) shows many vertices instead of extruded one. Distortion in edge plus vertex extruded from. Seen in retopology workflows.
Create simple sculpted mesh to retop over.
Create plane, enable snap to face, project + align.
Add subD modifier (3 levels in view), add shrinkwrap modifier, pick sculpt, above surface, offset .01
Align plane over sculpt as in retop workflow.
Extrude vertex, edge shows weirdness.
(pushing through and creating full face, corrects error. unfinished faces all show error.
Included .blend file. easier to see than explain. also side by side screenshot (left side = 2.79, right = 2.8).