The issue here is the software is working as it was told to.
There is a path to the third monitor from the main computer. Now why would it act like that I imagine you will ask, it is because Multiplicity is designed to allow you to have a computer turned off in which case it would skip large areas in the grid to find the next available machine, or you may have monitors which are aligned say right which you would still want to be able to access.
In this case it is not another machine it connects to, but a monitor.
Unfortunately this has never been reported to us before which is why it has not been addressed. I would imagine the number of people who have intentionally misaligned monitors like this is pretty small hence it never having come up before.
We will see about tweaking it to have a setting to ignore large gaps in this situation for future updates.
Thanks for letting us know.