Does water pressure drop when opening a faucet?
The PRV won't always keep the pressure from dropping below the pressure it's set at - in this case 75 psi. Its primary purpose is to keep the pressure from going over the set limit in the event of pressure surges. It's a safety device.
The pressure coming into most homes (curb pressure) is usually anywhere from 120 PSI upward. Usually the closer you are to the water supplier the higher the pressure. This curb pressure is generally too high for most faucets and other domestic water controllers and if it isn't reduced can cause damage and leaks.
As more faucets are opened the pressure will drop since the water has more outlets to flow through. The PRV will compensate for the pressure drop to some degree by allowing more flow but there is almost always some drop in pressure for various reasons. Sometimes this can be partially corrected by replacing the PRV, resizing water supply lines, or simply cleaning out hard water deposits or debris.
However, it would be helpful to know the curb pressure in your situation. The PRV cannot provide more pressure than what is available.