Okay, this seems like a real bug!
When the closed surface is subdivided in edit mode with smoothing factor = 1.0, it seems to look differently comparing to the result of Subdivision Surface modifier.
See the picture:
Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce RTX 2080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 435.80
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-24 14:22, hash: rB507ffee6e1f4
Short description of error
[Please fill out a short description of the error here]
Exact steps for others to reproduce the error
[Please describe the exact steps needed to reproduce the issue]
[Based on the default startup or an attached .blend file (as simple as possible)]