I open one of my Anaplan boards in NUX. On a board grid I select the "..." menu, then the Source module.
A new tab opens with the source model in the *legacy* model builder ux and also does not open the specified module.
With step #3 above, the new tab opens in the new model builder experience since I have it enabled. Also, ideally the source module would automatically open instead of me having to find and open it myself.