What is strange to me also is that, after moving that vertex with REFLEX ON, those new edges pointed by the black arrows appear in SMOOTH mode, but in FLAT mode they are not present (the smooth and flat mode not consistent with each other it looks)
The issue was that your edgeloop along the reflection plane is not perfectly planar. Remove reflect and and add it again to fix the extra edges getting added when you move a vert along the X axis. See the video attached where I show that there is a scale handle in Y on the Gumball when selecting this edgeloop and the GB is set to by object. If it was planar, there wouldn’t be a scale handle on the GB. Compare this to no scale handle on the GB after removing and adding Reflect again. Note that I used Shift held down when defining the reflect plane from the vert to make sure it was straight with the X axis. Then when you edit a vert along the reflect plane in the X direction, you won’t be moving at a slight angle, this was causing the extra edges.
I know this issue well and it causes me a lot of troubles. Succede that in fast transformation with the gumball I move verticies/edges away from the reflect plane by minimal distance without notice that in low poly view and it is annoying to correct this mistake many times afret several modeling steps.
Is there a way to lock verticies/edges on reflect plane to avoid this issue??
You can’t keep edges and verts along the reflection plane but you can use the Distance snap style in Reflect and set a value to account for edits that leave the plane. This feature is sometimes called clipping in other apps that have symmetry or mirror modifiers. It will auto merge verts within a distance but it can also make ngons when you don’t want them. This might help but I prefer to have an edgeloop along the reflect plane and only edit it in plane using the GB. It’s helpful to use the X or Y reflection plane options so you know you are aligned to these axis and then have the GB set to Cplane/World.
not really happy with this, can you please add an “option” to keep or not to keep?
in my mind if I selected some verts as reflection plane, moving one of those is like to change the reflection plane.
(so sorry, my english is not good enough to explain better)