Dear Mohsan Ali,   It ...

Dear Mohsan Ali,

It could be basic however would like to point out (sometimes sampling points's selection plays a vital role); 

Please check if your pH meter is calibrated (must be installed at Product of RO or RO Product Tank)

Please check your pH meter is calibrated at Receiving End means if it is coming into a Reservoir (20km Away).

Check your RO Product Tank Reservoir if not having any infiltration of the ground as instantly coming out of your RO pH might be around 6 - 7 and you might be measuring at that point but look into the Product Tank (reservoir from where you are pumping towards 20km).

Check your Receiving End Reservoir (20km Away) if it is not having infiltration or leakages. 

Check pH just from the pump discharge from RO Product and pH at the reception tank inlet pipe before entering into the reception tank.

The above checks will ensure if we need to look into the pipe line or not, first ensure these points. I strongly feel your sampling point  will play a vital role. 

If all are Ok on these, then there must be Pressure Release Valves/Surge Release Valves on elevated points of pipeline due to topography , please check any of these points are intact or are not the reason for any infiltration.

Please check further detailed analysis of water for RO Product, RO Product Reservoir, Before Entering Reception (20km away) and from Reception Reservoir; 

pH

TDS 

Conductivity

Hardness 

Alkalinity (Methyl Orange Alkalinity)

Phenolphthalein Alkalinity

This will help you if other parameters are also not changed.

Hope you will find now the main cause and solve this issue. Please share your findings.