Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.86
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-26 15:34, hash: rB79ce2054d436
Short description of error
When Subdivision Surface modifier is added on mesh with loose edges,
it behaves strangely.
The attached image shows how it looks when Simple suvdivision Surface modifier is added on the icosphere with loose edges
Exact steps for others to reproduce the error
Add icosphere, and edit it.
Delete the faces by Delete Only Faces, and only loose edges remains.
Then, add subdivision surface modifier and set it to Simple Type.
The mesh seems strage.
It seems that z-component of subdivided vertices are set to be 0.
I am not so sure, but I guess that only x- and y- components are subdivided properly ?
(Subdivision surface modifier on loose edges have changed for fixing of recent bug report T67599.
So this may come from that. )