Pipe flow goes negative

In the model above, I am expanding an existing water system to a new building.  Per the results of hydrant flow tests, the available flow rate at R-4/PMP-4 is 1,060 gpm and at R-3/PMP-3 is 840 gpm (per a prior question, I am using an approximation method to model the hydrant flow tests from an article that was provided Bentley). My question: why would the flow in pipe P-7 go negative and why would the system try to pull more water from R-3/PMP-3.

See attached for two clips from the software.

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