Not syncing preexisting user changes

Hi,

Having some sync issues. Not even sure where to go about debugging.
1. Installed User Sync (and the base plugin) on localhost.
2. Updated a user’s meta via a custom frontend form (uses update_user_meta action).
3. Plugin detected user change, and synced it to Mailchimp.
—So far so good —
4. Looked at the Mailchimp List, and noticed that the email used was the email with which the account was created ([email protected]). That email address had been changed to a different email ([email protected]) before the plugins were installed. All the other fields (name, and the custom fields I mapped via the plugin) synced fine.
5. Updated the user meta again via the frontend form (changed the custom fields). The plugin saw there was a change, I waited for it to sync, checked Mailchimp, and no changes had been made.
6. Updated the user meta again (changing the built in user_firstname and user_lastname)… and no changes were made in mailchimp.
7. Updated the user meta again (changing the email from [email protected] to [email protected]), still no change.

Using the latest version of both plugins (4.1.12 and 1.6.4 for User Sync), and latest 4.9.1 of wordpress on localhost running PHP 7.1.2. Disabled all other plugins first (the frontend form is something I coded into the theme).

Not even sure where to begin debugging this one. Any suggestions?
(both to why it synced the orig email address, and why it isnt syncing changes)

TIA!
(PS: Is there a way to force trigger the sync, instead of waiting for the plugin to do it itself – mind you, not the manual sync for all users, just want to push the changes the plugin detected.