I forgot to mention that I set the app to automatically release once approved and it's available in the app store.

I have submitted a ticket. Based on further testing, I have determined that the promo codes only work if users have their profile set on the Romanian app store (which is my home country and the country I have my own profile set on). However, the app is set for global distribution in all countries and regions.

On my side, I can confirm that the issue seems to be resolved. The good thing is that previously generated codes that could not be redeemed because of this issue can still be used now after the fix.