In C4D, why does UL always choose two lines through cyclic selection instead of one line?

In terms of model, check whether there are any places to choose: trilateral faces, poles, interpolation faces, overlapping points, lines and faces. If it is removed, you can generally choose.

In terms of software, check whether the shortcut keys used conflict, whether the settings (loop boundary selection) and operation methods in loop selection are correct, and restart if it feels like a bug.