Is it possible that you also selected the NDI device as output #1 in Startup Configuration?

There are two possible approaches:

a) If you use the Device Video Out module for NDI then do not assign NDI to any output index in  Startup Configuration.

b) The more convenient usage is not using Device Video Out module at all in your scene. Instead simply assign NDI as output #2.

This is highly advisable if you use NDI as you main output.

Thank you I have solved the problem using the startup setting you suggest .

But should I use sync with ndi like SDI?

Yes, if you want a stable framerate that is in sync with your NDI output.

Otherwise the system will run with the framerate specified in Preferences.