I found the issue. Turns out if the check for blocking option (BAI) is set these stairs stop working. Same for the ones on the cruise ship. If I turn it off it works again.
But that might lead to an other issue and that is that sometimes in the jungle a bush spawns at the ramp and if you have a tank in the front you are stuck. I tried to blow it up but it didn't work so I had to leave. It has happened a couple of times.
Out of sheer curiosity I decided to investigate possible causes of the ramp bush. Now I've managed to narrow down this particular case as a combination of the Osprey craft and the MOUNTBD_TROPICAL LZ blocks, particulary MOUNTBD02.
While using OXCE New Battle fuctionality I noticed something, the plant-bush in question is visibly overlapping the lowest ramp section and while viewing the mapblock directly with mapview2 I further noticed that the bush is placed in the tile's floor slot, not the content slot. Had it been placed as content, the ramp would have removed the bush entirely resolving this issue. In fact, the mapblock has a leafy bush to the lower left of that same plant-bush that is effectively removed by the mid section of the ramp. Additionally, this plant-bush can be found placed in other mapblocks using either the content slot or the floor slot depending on the mapblock being viewed. While it is not unusual to place some sprites in a slot different than its intended one when trying to fit multiple sprites into the same tile, this is not the case here since nothing else is being placed into that tile besides the plant-bush.
Anyway, this particular instance should be fixed by merely changing the slot used by the plant-bush in that mapblock to the content slot instead of the floor slot and filling the floor slot with any of the sprites available designed to be used as such.