Original XCom had a number of issues related to pathfinding. OXC works differently, but there is a thing, which make it slightly complicated: going upstairs without switching a level.
In original XCom they just go straight path for current level, and if this path will go through elevating tiles - unit will go different level. This was used for going upstairs without switching level up (this switch happen automatically when unit goes other level).
If it will be simple optimized a* path, it will go exactly where you pointed, but you will get tired of switching levels for going up or down. So to emulate original-like behaviour for this issue they've decided to autochange level at certain conditions.
If I remember correctly, this condition is (at least) "unit stands on elevated object (stairs)". So why it goes upper level.
They could add another condition, like "work only when pointing tile, adjacent to elevated", to avoid such issues, but in this case it will become something people haven't used to. And if you wish to go tile adjacent to elevated but lower level, it will go up.
I've deeply thinking of idea of how to make something logical and handy as original (for going upstairs without switching levels), and nothing worth of implemening crossed my mind.
You see, there are two options of situation when your unit is near stairs - to use them or to use not. How program can predict what do you actually want?
I can suggest you to rely on path hightlighting (though Daiky still haven't made it "hover" highlighting).