From someone who posted on the other site
Thank you for contacting Wizards of the Coast Customer Service! The issue with the Mastery Pass not granting Uncommon ICRs on Levels 70 and higher has been reported to the Arena development team. Players who are missing Uncommon ICRs will be receiving a grant sometime in the near future. Thank you for your patience while this is being worked on.
Mastery pass ICR have a 1/20 chance of upgrading to rares ICR, so people at level 90 were also likely to receive a rare ICR. So even granting us only uncommons ICRs would be bad tbh.