I am facing a similar issue. Lost steering authority at low airspeed making taxiing impossible. I step fully on my Saitek rudder pedals, I see the rudder control surface and nose wheel deflect, but the aircraft refuses to turn properly; super slow response when taxiing slightly fast but almost not authority at normal taxi speed. This problem started a couple of days ago with all GA fixed wing planes, and I haven't really installed any new add-ons. I've done everything possible, and out of frustration deleted and reinstalled X-Plane 11 last night and tried again to no avail. The hardware seems to be working just fine considering that I see the nose wheel turn, but there is something not quite right anymore. The problem seems to disappear when I gain airspeed, such as rolling down the runway, then I regain authority but at the moment I'm only able to taxi with differential braking as normal steering is barely doing anything!
I've searched endlessly last night for a solution and couldn't find any, I'm hoping there is some kind of a fix or pointer to what the problem is.