The square Hebrew script writes five of its letters differently at the end of a word: kaf, mem, nun, pe and tsade each have a final form. Samaritan has none, and a reader coming from Hebrew usually assumes something has been dropped. Nothing has. The finals are an innovation of the script Hebrew adopted, not an inheritance the Samaritans lost.
The sequence of events is this. Paleo-Hebrew, the Israelite script, had one shape per letter. After the Babylonian exile Jewish scribes took up the Aramaic script for writing Hebrew, and it was inside that script, as it was written in ink with a reed rather than cut in stone, that certain letters developed a long descending tail when nothing followed them on the line. Over time the two shapes were standardised as separate forms and the distinction became part of the alphabet. The Samaritan community never made that switch. Its script continued the Paleo-Hebrew line directly, so the conditions that produced the finals never arose in it.
This is the clearest single test of the relationship between the two scripts, and it settles the question people most often get backwards. If Samaritan were a decorative variant of the square letters it would have inherited the finals, because they are part of what the square script is. It does not have them because it is not downstream of the square script at all. The two are branches from one root, and this is the branch that did not change.