Unable to add OrgID as co-administrator

We have an Office365 AAD tenant, and two older Azure Subscriptions with Microsoft IDs as account & service administrators.

I'm trying to add OrgIDs from the Office365 directory as subscription co-administrators in the Azure Portal, but I keep getting: 'You cannot add this user as a co-administrator because the user is not in the same directory as the account administrator of this subscription.'

I've definitely added both the account and service administrator Microsoft IDs to the AAD tenant as Global Administrators. I'm unsure how else to resolve this.