Keep in mind that the height returned by SampleHeightMostDetailed is relative to the WGS84 ellipsoid, which is not always aligned with the terrain’s surface. You can always convert this WGS84-relative height to something else, like its equivalent in Mean Sea Level (MSL)
We currently don’t have any way of calculating MSL height in the plugin, but there are some posts in the forum that give suggestions, like so: