User profiles get stuck in update loop, even if profile wasn't updated

When this plugin is connected to MC via the API and a callback, the plugin appears to constantly update user profiles, even though no profile changes were made.

Steps to reproduce are:

- Setup Mailchimp plugin
- Connect via MC API key
- Enable callback both in MC & this plugin
- Create a user on MC
- User syncs to WP system as expected
- Check log for multiple repeat requests to update the plugin.

I notice that since enabling the plugin on a test environment, my site keeps giving 504 error (not enough resources) and I'm concerned that the plugins constant updating contacts is the cause.

Our host is WPEngine, latest Wordpress version, tested with the same behaviour on single and multisite setups.

Here are some screenshots showing the log and how the plugin is configured (note that I removed our MC API key for the settings screenshot).