How is that possible when it carries no load?
I'm assuming both 4-wire wye sources are active and parallel connected as indicated by the dashed lines on your diagram.
As an example, let there be perfect phase alignment between the two sources with no loads on the 3-wire line outputs. If the open circuit L-N RMS voltage is identical on all terminals of both sources, then no current will flow on any interconnecting conductors when connected.
Now if the L-N open circuit voltages are reduced on source 2 but they are still balanced, equal RMS currents will flow between the three phases (L1,L2,L3) of source 1 and those of source 2. However, the current on the neutral will still be zero because both sources are balanced.
Now consider the case where the open circuit voltage is reduced on only one of the L-N phases of source 2. In this case current will flow from source 1 to source 2 on that phase but not the other two, and therefore an identical return current will flow on the neutral conductor. So in a more general case with sources that are not perfectly balanced there can be current on the interconnecting neutral conductor, and therefore voltage drop across it..