O365 Groups Prefix Policy

I think this is via design. A single group naming policy exists in the Exchange Organization configuration that is applied to both Exchange groups and Office 365 Groups. Ideally, in a hybrid environment, the same naming policy should exist in both environments to ensure consistency of group names for both on-premises and cloud users. To ensure consistency, the policy is applied to groups that belong to the on-premises environment as they are synchronized into Office 365. The reason why you have a problem is that you don't have the same naming policy defined on-premises or you have some groups that were created on-premises before a naming policy was in place. Or perhaps that you simply didn't realize that the naming policy applies to other groups than Office 365 Groups!