Connect context parameter for username macro parameter with multiple values is returned empty

Issue Summary

For Connect apps that have opted in to GDPR, macro parameters of type username are not properly resolved as iframe URL context parameters when multiple users have been selected.

Steps to Reproduce

1. Install a Connect app that defines a macro with a username parameter.
2. Insert an instance of the macro, selecting multiple users for the username parameter.
3. Load the page containing the macro.

Expected Results

If the app has not opted in to GDPR, the context parameter for the username parameter resolves to a comma-separated list of usernames, and an additional context parameter with the ".accountId" suffix resolves to a comma-separated list of account IDs.

If the app has opted in to GDPR, the context parameter for the username parameter resolves to a comma-separated list of account IDs.

Actual Results

The context parameters that should be lists of account IDs are empty.

Workaround

Opt out of Connect GDPR API migration.