Same issue here,
In my case, i already have a normal account and i just modify my mail to my university’s mail.
The new mail was approved for student discount. However, i got this notification too
Cannot use Promo Code: Promotion only available to students
This problem is not new apparently.
https://discourse.webflow.com/t/student-discount-doesnt-work-if-i-switch-to-my-student-email/195000