If not using OSU, are you downloading an OFX or similar file and importing that? If so, my guess is that the name of the account in Quicken doesn't match the exact name in the OFX file. Or, if you edit the account settings for your MG account in Quicken… does it have the full account number shown there? Maybe the mapping is looking for the account number.
To track this down, compare every account setting for the newly created account vs the existing account to see where they differ.