I am using Microsoft Word 2016. And I am facing an issue with my Arabic fonts and other Arabic system fonts ( Times new roman - Tahoma - Arial …) when I use ( justify low) option. There are some spaces and gaps between Kasheda.ar and the next glyph ( after printing). please see image
My version of Calibri does not contain Arabic. Perhaps take a look at the kashida-ar and its relationship to the nbspace. And of course, it is also luck, because it has to stretch very far, it may put some of the weight on word spaces, not just the kashida. Some apps offer extra Arabic justification settings for this, but I don’t think Word does.
What I can say for making fonts work in Word on Windows:
Make sure you cover Win1252 completely, including nbspace.