PRV valve installed in a way that it is subject to flow in the reverse direction it should normally works in. I know that some PRVs come with a check valve feature so this shouldn't be a problem for them, my question is what happen to the valve if this feature is not their and the flow is reversed?
A 100mm PRV >may< have an external sensing connection.  If the external sensing connection is connected to the conventionally accepted downstream side of the valve then the valve will work nearly normally.  There will likely be some offset due to the inverse sympathetic ratio being inverted, but your operators may not have noticed or just figured they have to set the valve a little off the desired set pressure to get proper function. 
If the valve has intrernal sensing,and it is installed backward,  then it will open whenever the upstream pressure is low, and close (probably slamming abruptly) whenever the upstream pressure exceeds the set pressure.  If this is the case since the valve is spring-loaded open, you may never have reached the set pressure.  Once it closes, you may find it necessary to vent the system upstream to get it to reset. 

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