Blending paths with brushes and width profiles works wrong.
Create two lines.
Apply art or pattern brushes to both of them.
Apply width profiles to both of them.
Ai will generate steps that have uniform profiles instead of intermediate.
Scatter, calligraphic, and bristle brushes won't let you edit the width profile. I did however test with two calligraphic brush paths with different pressure curves and that was ignored for the intermediary paths. Please test this with bristle and scatter brushes as well.
(this is essentially a reopening of https://illustrator.uservoice.com/forums/601447-illustrator-desktop-bugs/suggestions/41634754-blending-paths-with-width-profiles-works-wrong with a feature that was missed when originally implementing it, as such I just copied and edited Egor's description :) )