This is functioning as designed.
Since the User was created within a User Group, the User cannot exist independently outside of the User Group. Spectrum has to add the User Group to the new landscape and hence this landscape name is added to the User Group. So when the User Group is added to the new landscape and the landscape added to the User Group, the landscape is added to the other Users associated with the User Group.
If you need a User to have the same access and privileges as an existing User Group but need to add a new landscape for that User, you will need to create a new User Group with the same access and privileges as the existing User Group but different landscapes.