It may have something to do with the 'Void Fill' settings in the tool? If so, which setting would fix this?
Or maybe this from ESRI's LAS Dataset to Raster tool help: "Including a study area boundary as a clip constraint in the definition of the input LAS dataset is highly recommended. One reason is to prevent interpolation from occurring outside the real data extent of the survey." If it's this, how do I make a clip constraint in the LAS Dataset?