Strange behaviour of nrf52840-DK with NUS

[pdcampo]

I have some experience working with nrf52840-dk and the SDK working in custom projects taking as initial point the different examples of the SDK. 

The problem is that I have just bought two new nrf52840-dk and these board have a strange behaviour that the rest of my dk boards doesn't have with the standard NUS example (SDK version 15).

With the new boards if I write something (ending with CR and LF) through a uart terminal the message doesn't print in the other terminal. It only appears when I write a new message but this new message only will appear with a new one. 

This doesn't happen with the rest of my boards with the same hex application and softdevice. 

Any idea that bring light on this?

Many thanks. 

[pdcampo]

I also have checked that the version within the Debug chip is also the same between the new boards and the old boards. Any idea? Thanks