I found out that the three UPS voltage sniffers located in DAQ room had been erroneously plugged the same multi-plug at the back of the ENV rack. Who did this action probably inteded to "clean up", but indeed the three sniffers must stay plugged multiplugs belonging to different racks in order to correctly monitor the R, S, and T phases of UPS voltage!

So, I plugged them back to the original plugs. Now the signals look correct (Figure 1).

Note that these sensors are also important because are going to be used for 50Hz feed forward.