Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce 940M/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 376.54
Broken: version: 2.80 (sub 72), branch: master, commit date: 2019-05-25 23:18, hash: rBf18373a9ab1a
Short description of error
When using the mirror modifier and subdivision surface, the mirrored edges don't get smooth, even when the modifier is applied. (with the subsurface at the bottom) settings like clipping, merge etc. all don't solve the problem.
Exact steps for others to reproduce the error
1-create a flat circle with 16 vertices, with a face and a hole in the middle;
2-delete 3/4th of the circle, leaving a quarter;
3-activate a mirror modifier and tick the axes so you'll have a complete circle with a hole in the middle
4-activate a subdivision surface modifier (after the mirror)
5-go into edit mode, and extrude the circle
Normally, you would have a completely round donut. However, at the mirror axes the subsurf doesn't work and there's a sharp corner. It does work however, if you extrude the circle before activating a mirror modifier. Note that I haven't applied any modifiers, and the two objects are completely the same. So, according to the non-destructiveness of the modifier workflow, the objects should be the same, but they're not.