It seems like you have a problem then somewhere between the USB port of your computer and the tape deck. Or, maybe there's some configuration of the interface inputs to Audacity L&R channels that has been messed up, e.g., via a Windows update.
So, first, check your audio configuration in the Control Panel and/or Audacity to make sure that when the interface is the input device, it has 2 channels input, and in Audacity, those two channels are assigned correctly to the L&R of the stereo track or two separate mono tracks if you record that way.
With that out of the way, and assuming it doesn't change anything, since you've tried different tape decks, it is likely either the cable or the interface. If you used the same cable with every deck, that has not been ruled out. If you used different cables, then it has less likelihood of being the problem. But, if it's the same cable, then simply swapping the L&R channel at one end or the other should switch the channel you see show up in Audacity. If the it's the R channel always being recorded, regardless of whether you connect L or R out from the tape to L & R or R & L, then it's the interface. If it moves, it's the cable, i.e., that means the cable is only sending one channel to the interface. If the channel sent to the computer is always the R, even when you know a good signal is going into the L side, then the interface is not working properly.