I have a project in N5.5.6 with a loudness of 0.3 LU (Nugen loudness meter)
When I open the same project in N6.0.5, the loudness is 0.7 LU (Nugen loudness meter and Loudness track).
It happens with more projects, in N6 they are louder than in N5.5, approx. 0.3/0.4 LU.
To be sure I exported the ‘louder’ N6 mix and checked that in N5.5, and the export is also 0.7 LU louder.