Unfortunately, the plugin Checkout Fields Manager for WooCommerce (by QuadLayers) is not compatible with Fluid Checkout.
We’ve tried multiple times to add compatibility with it without success. This problem is due to how Fluid Checkout saves the user data during checkout and the way Checkout Fields Manager for WooCommerce (by QuadLayers) processes its custom fields or changes to existing fields.
As a workaround, we suggest you replace the plugin WooCommerce Checkout Manager (by QuadLayer) with the similar compatible plugin Checkout Field Editor for WooCommerce (by Themehigh).
We’ll keep a task in the backlog to add compatibility with WooCommerce Checkout Manager by QuadLayers in the future, however, and to be honest, it will take a long while until we consider working on this task again.