I popped open my 2019 run of the Legacy ATSF 3751 class Northern to check the gearbox. I took it apart to lube it when I first bought it but forgot what it looked like. There is a spacer next to the worm wheel to keep it centered. You can see it to the right of the worm shaft in this photo.
It appears that the issue of the worm wheel shifting may be limited to the early Legacy K4. So either the K4 gearbox was designed without the spacer to keep the worm wheel centered or the manufacture forgot to add it when assembling. One would have to open more former K-Line tooled locomotives, with the forward facing small 385 style motor with the worm pressed to the motor, to see if Lionel corrected the gearbox issue in the later runs.
But, based on this thread, it sounds like the Legacy steam equipped with this multiple gear setup in their gearbox and a worm shaft separate from the motor aren't affected by this issue. So far the gearboxes on these locomotives have had a spacer to keep the worm wheel centered under the worm shaft.
