soft selection - edge distance: error while selecting

[12harrypotter34]

Hello everyone, I can't explain why this error keeps popping up, any help super appreciated!  (photos attached)

I have imported in 3dsmax a shape drawn in Autocad, perfectly converted to Edit poly, and now I just need to bend one end upwards (maintaining a linear trend while height changes). Imagine it as a track that must always remain parallel to the ground level, even when it becomes a climb or a descent.

Best way for me would be a Soft selection with Edge distance active: selecting the last edge/pair of vertex, play with falloff/pinch/bubble until ok, and only then move the whole selection upwards.

The problem is right on the very last blue vertex of the soft selection! The last part keeps getting selected with a difference of several vertex, which then causes a really bad difference in elevation.

I tried resetting xform, converting to editable mesh/poly, changing lots of times those soft selection parameters, and now I'm running out of ideas. Any ideas about what could make it work?

Any different workflow to achieve the result would be ideal as well!

Thank you in advance 😊

[RobH2]

Can you upload that shape? I'd like to play with it. But, it looks like maybe there are some coincident verts that the flawed area that aren't welded.