Unfortunately this is a limitation when using the Generator > Limit to ID
feature on curved splines. Internally each spline section is considered separate so it cannot compute the curves required to create a continuous deformation where the two IDs adjoin.
However, with a little work it is possible to recreate the style using a different technique that doesn't use this feature.
In this approach we can use the Conditional node instead of limit by IDs to detect the ID of the spline segment and add the relevant segments only to the appropriate spline sections. In order for this to work all the vertices within a spline section with the same ID should be either curve or bezier, with corner or bezier-corner reserved to mark a material ID change. You will also find using this technique that evenly mode does not work well, in fact I've removed it in the example, which can result in some alignment issues. Also you can no longer use the Start/End inputs to add the ramps at the beginning of the drop curbs so I've recreated it with composed and conditional corners, but it is a little trickier.
If I find another way to achieve this I'll post it here.