When stacking star trails in Nebulb you have two options for the representation of the ground part:
Well, if you select the Sigma-clipping option for the ground, the stars reflected in the water will not appear (neither the stars, nor the moon, nor anything that passes through the sky and is reflected in the water).
This is because Sigma-clipping stacking is really effective in removing moving objects, so if those objects are reflections on the ground part then those reflections disappear during stacking, as you can see in the image below where there is no reflection of the moon on the water (even though the moon was present for some time and reflected intensely on the water):
That said, it is perfectly possible to make the reflections in the water appear. To do this you simply have to select the Maximum image for the ground, as you can see in the following image:
Both images have been processed by Nebulb during the same stacking, the only difference between them is the image I have selected for the ground (Sigma-clipping or Maximum), something that can be easily selected at the end of the stacking and, that way, you can keep the version you like the most.
Another use case in which it is often interesting to use the Maximum image for the ground is when there are trees that let the stars glimpse between their branches. In those cases the maximum image for the ground usually gives a touch of greater realism to the final image, since the trails of stars have continuity between the branches of the trees (something that is not achieved by selecting the Sigma-clipping image for the ground).
In any case, it is always best to try both options and choose the one that looks best in each case.