Hello @Josie4
I understand that you have a question about rubrics when viewed on the mobile iOS app for students.
I tested this on my end as a student in my sandbox environment and I was able to view the long description used on a rubric without any issues.
What I found is that If the students are looking at the assignment on the mobile app for iOS, they would select the button for Submission & Rubric. From there they can tap on the tab for Rubric, and this opens up a display window for the Rubric Criteria, a link to the Criteria description, and the possible Criteria Ratings. If the students tap the blue Description link, it then opens up what you put in the long description.
If you students are not seeing this, then it might be best to have Canvas support take a closer look.
Hopefully this helps!
-Colton