Yes, you are right. All question to Honda and Garmin are without answer.

I have bought it in Europe from Honda in 2015. The navi system is the same as in Goldwing from USA. My girlfriend bought it from USA and I loaded maps to her navigation.The navi system accepted more than 4GB of maps. In this time I have 3,7GB with no problem. The 32 GB navi not accepted.
As I wrote. Several years I downloaded maps from garminopenstreetmap.nl. Ii was without problem. I am sorry I do not know difference between NT and NTU.
I would like send you printscreen of original Honda SD with name of files. I cannot add picture to my post.