Your Boss z axis limit switch is probably working just fine.
I have a Bridgeport Boss 6 Rigid Ram and I believe I may know what is going on.
If I am reading your post correctly, what you have found is if you go too far down the z axis limit switch trips, halting the mill.
When you go too far up, the z axis simply "bumps" up against it's uppermost travel and doesn't trigger anything. You could "bump" up forever!
This is exactly right. Nothing wrong here. The Bridgeport Rigid Ram mechanically allows this and is designed for this exact action.
The reason this is allowed is because there does exist a z axis uppermost limit switch of sorts, but it is a reference switch and not a limit switch.
This way, when you click the "Reference All Home" button in Mach3, the x and y axis goes to zero while the z axis rises till it trips the z axis reference switch. You will notice the z axis actually rises a bit above the reference switch and lowers to just the right height. It's a wonderfully accurate way to set the z axis height, specially when it comes to tool changes in the tool table.
This is a good thing. I use this feature extensively to raise the z axis to a repeatable consistent position, then set the z axis to zero so the reference position and axis positions are all zero, then measure the tool length down from that point to the top of the material. If you set it up so that all your tools have a negative height (-2.11, -1.65, etc.) then you can program your tool table for tool changes.
It works perfectly.
(As a note, the z axis should be 1 - 2 inches below the uppermost limit when referencing home otherwise it cannot rise above the reference switch because it's already above it, and therefore the z axis cannot reference. Simply lower the z axis and try again.)