Thanks Giomen!

I re-read the first post here, the known limitations and issues of the current version explain what is happening perfectly:

1. All the JNX levels with the scales below 864 will be mapped into a lowest zoom level in IMG, it becomes visible from 200m and closer in the devices. If there is more than one level in your JNX with small scale value, the converter will only use one of them - one with lowest scale value (i.e. the most detailed one).

2. JNX levels are displayed in more than one zoom. For example, a level with the scale value 4777 is visible from 1.2km down to 5m (if more detailed levels are not present in the device). In IMG, in contrast, the level is only shown in the specified zooms, so, to emulate a JNX way of displaying the tiles, they should be placed in all the necessary zooms explicitly.
Current version of the converter does not provide this emulation.
So I guess we have to live with it until the application is updated to solve those issues.

It is still usable for me on the 5m to 200m setting, but not ideal.

For device with JNX support the pure JNX format is more convenient.
AFAIAA, I have to hack my firmware though to use JNX. I'm not sure if that is more convenient, and it is not as safe. Also, it means I can't share maps I made with friends unless they have also hacked their device. Correct me if I am wrong about this :-)