User import does not add roles from "Additional roles" when updating existing users

The scenario:
My system contains existing users with various roles. I am importing a list of users (8,000) with a role of 'Donor' which needs to update the existing users with this donor role.

Settings applied for user processor are:
Update existing users: Checked <<< this is the difference
Skip hash check: Unchecked
Status: Active

Additional Roles:
Checked Donor (a custom role for us)

Field mapping
Used Email Address as unique
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The problem:
The import is not setting the additional role upon updating existing users. It just leaves the existing roles in place and does nothing to set the 'Donor' role.

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This ticket is similar to but not the same as https://www.drupal.org/node/1894542. His ticket is an issue for replacing, while mine is an issue for updating.

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I have tried using the module https://www.drupal.org/project/feeds_ridmap but it just does a replace and my existing users lose their pre-existing roles. So feeds_ridmap is kind of like just checking the option for "replace existing".

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I don't have enough time to debug this, but if someone already knows and can tell me where the source of the problem is with regards to updating roles in the feeds import, then I am happy to try to fix this when I find some spare time.

Best, Howard