Unusual low pressures

Hi,

All of a sudden a certain model scenario is not running without problems. Here is the rundown of what happened. I will secure upload the model file. 

I started with the Base scenario "P2 Max Day EPS ADD DIST", then I created the Child scenario "P2 Max Day SS FF @ WTP" with four new alternatives:

I wanted to add a demand to node J-607 at the north end of the model where the Water Treatment Plant was of 1,500 gpm to simulate a fire flow steady state analysis here (I did not want to run an actual fire flow from a calculation standpoint). 

The point of this fire flow analysis was to see if the system could provide a flow of 1,500 gpm at J-607 if the pump were to lose power, thus I used a topology alternative to make the pump (PMP-600), tank(600) and connecting nodes/pipes at the WTP (being simulated by a negative demand node J-600) not active. I also used this alternative to create a new connection to the system that bypasses the existing PRV-228 near J-32, thus allowing backflow from the system toward J-607. I also turned to not active (topology) the line that would have connected the water treatment plant J(-600) to the distribution tanks (T-200 and T-1) if the pump was running. In other words, in this system, the WTP has lost all connectivity to the distribution tanks. In this system there are also master meter locations simulated by a reservoir, FCV combination which can allow flow in to help meet system demands.

Then, I used the demand alternative to add a 1,500 gpm demand at node J-607. (Technically 750 but there are active demand adjustments in the calculation options)

Nothing else was changed, therefore only new changes were made to this new fire flow scenario through its new alternatives. BUT, when I went back to the "P2 Max Day EPS ADD DIST" and ran it, all of a sudden the dedicated line to the tank had extremely negative pressures. I am not sure what to make of this as it was running just fine before and it seems more like a quirk than a true negative pressure. 

For comparison, scenario "P2.4 Max Day ADD DIST" is very similar to "P2 Max Day EPS ADD DIST":

The only difference between these two is a slightly different target pressure for PMP-600 and a different water treatment plant production (negative demand). I also created a similar fire floe analysis under the "P2.4 Max Day ADD DIST" and both this ("P2.4 Max Day SS FF @ WTP") and ""P2.4 Max Day ADD DIST" work fine. So I am at a complete loss and could really use some help. I am supposed to deliver a working model soon and now this has happened.

Thank you,

Nastassja