Negative payload restriction R1

In plain image it looks like this:

It happens when I close couple of flights on the route and want to reopen them with different aircraft. It doesn't affect the actual payload after adding some flights, and after doing so everything gets fixed by itself. 

The actual number seems to depend on capacity of the aircraft being put out of flight plan but it looks like it's not related to the aircraft we may choose to launch (here both DC-10 and 747 have the same payload 'restriction').