It seems that the signal going into the select pin of the multiplexer (mux) from the testbench is connected to either GND or VDD. However, instead of getting a voltage of 0V or 1.8V on the select pin, you are observing values in between. When directly connecting GND to the select pin, the mux operates as expected. However, when providing GND through sel_pin[0:1], the mux does not select the correct values, leading to undesired behavior. The reason for the select pin exhibiting these unusual values is not clear. I appreciate it if you could provide insights into the cause of this issue.

You see the MUX standard cell does not have power (VCC and VSS) pins. The power connections as well as the p-well and n-well connections are set by instance attributes. Click the mux cell, press 'q' to edit its attribyutes and set the values:

VGND=VSS VNB=VSS VPB=VCC VPWR=VCC

VGND is the groudn connection, VNB is the n-channel transistors body connection (almost always set to the same node as VGND), VPB is the p-channel body connection (often same as VPWR), VPWR is the power supply, Set the right hand side to your power supply node (VCC, VDD, VCCD, whatever) and ground node (GND, VSS, VSSD, whatever).