Can SRoute route to the line shape pin of Vdd and GND?

In my own cell library, the IO and power/ground ports are all the line labels. I prefer that in the lef file, they are also line label pins. For the signal pins, it works fine by assigning them as line shape pin, but for the power/ground pins, if they are assigned as line shape pins, the SRoute did not route at all and the Vdd and GND are open nets. Even if I change only one cell's power and ground pins to square pins that the SRoute will route normally. But I cannot explain why does it act like that?

 In the soce user guide, it only said that "it connect power and ground nets from end-to-end, terminating at the power rings."

I checked the solutions for open nets in soce user guide, it suggested that it might be problems with pins that have hot physical geometry, which I think the line label pin is of this kind. But if it is the problem, then, why does it route correctly with IO line label pins and also with power/ground pins even if there is only one cell containing square power/ground pins?

Can anyone explain the action of SRoute when routing power and ground nets?

Thank you very much!