Unfortunately Dorico can’t always correctly predict whether a frame is going to become overfull, and once it has decided on the casting-off result, it can’t change its mind once it discovers that it was wrong. Dorico doesn’t know exactly how every item on every system will be positioned until the music is fully spaced, and of course the placement of some items, like beams etc., is dependent on the spacing itself, so there is a circularity to this process: it can’t figure out whether or not the music will fit in the frame until it’s cast-off, but it can’t cast it off until it knows whether it will fit in the frame, and so on.
Dorico makes estimates before it casts off about the likely height of each system, but if its estimates are wrong, it can end up thinking that one more system can fit than actually can.
We don’t rule out coming back around to try and solve this circular dependency another way in the future, but it’s not something you should expect to change soon. For the time being, I suggest you simply set a frame break at the start of the final system on the first page.