Heyo.
I’ve noticed if you place a Reaper and a Zergling on a high ground, and issue a unit on the low ground to MOVE to the Zergling on the high ground, the low ground unit will path the long way around.
However, if you tell the low ground unit to MOVE to the Reaper, it will kind of “derp” and head straight for the cliff. This appears to be less consistent of a bug depending on the distance between the units and/or if the target unit isn’t idle.
Example: https://streamable.com/ttdsyy
This seems to be dependent on the target unit’s MOVER. The Reaper on the high ground uses the mover Cliff Jumper, and the Zergling on the high ground uses the mover Ground. I’ve tested with several different units and this appears to be the only distinguishing attribute.
I’m not an expert though and would love to be proven wrong.
Another example: https://streamable.com/q6vm9r
And one more: https://streamable.com/siv9qa
Really I’m just hoping this is a bug and is fixed so that my ability can work in my custom map 
But uh, we’ll see.