I just checked, both today and a little last night, on my N900 aircrack-ng is also showing length data as it should - for my family home network, I can also verify it's showing the correct length, so it's not like it's showing random numbers either. Saturn, when you say "the N900 version doesn't return" this data for hidden networks, how are you getting the airodump-ng output into your Cleven program?
airodump-ng -w filename wlan0