It's probably the vertical edge throwing it off. Unfortunately (for you), TINs can't have vertical faces (only one Z value for every X,Y coordinate). So it's skipping that corner. You'll need to introduce a slight slope to each vertical line in each section to get a better TIN.
The process you outlined (prop xsec -> DAT -> TIN -> exist xsec) could be shortened to just [prop xsec -> exist xsec] by just changing the properties of the proposed linework. Do you really need the TIN for some reason, or could you save yourself the trouble and just modify the linework from the proposed sections?