A FCV will act very similar to a negative demand in that EPANET will check what head is required to push the FCV setting into the system. The main difference is that the FCV will then check to make sure the head upstream of the valve is higher than the required head to push the flow.
If the required head exceeds the head required the valve will induce headloss such that the head downstream is exactly what is needed to push the flow setting on the valve and the valve will flow at the specified setting.
If the required head is lower than the head upstream the valve will error “Open but cannot deliver flow” and will allow the flow that can be pushed by the upstream head to occur, but it will be less than the valve setting.
Based on you noting the head is higher than expected, it is likely that the head is high enough to keep the valve from flowing at the full setting specified.
To resolve I would investigate the elements that could be raising the head higher than expected downstream of the FCV.
Possible causes you may want to investigate: (My apologies I can’t troubleshoot your model for you at this time)
NOTE: Make sure to set under Options -> Hydraulics to set the Status Report to FULL for best assistance when troubleshooting a model as this gives much more information to help you.
A valve may have an initial Status or control set to make the valve have a Status = Open which makes it act like an open pipe rather than an active valve.
Pumps are running too far to the left on the pump curve possibly due to low demand or having too many pumps on for the demand. This would increase the head gain of the pump and increase the discharge pressure
Look at any other factor in the model that is controlling the pressure for what might be boosting the pressure (and thus the head) too high.