Dynamics are acting strange

select the dynamic open the inspector (F8) and set the dynamic range to staff (stave in GB). The default dynamic range is the part or instrument which is all staves included in the instrument.

Note if you are just trying to make the melody come out in a natural way the way a real pianist would do by default anyhow, another approach that may be preferable is to increase the velocity of the melody notes in the Inspector. This is more flexible in that it also allows you to differentiate by voice, to account for melodies that cross between staves, to have the top note of a single chord louder than the rest, etc.