I noticed AdminOrderCreationPlugin has its own CustomerProvider https://github.com/Sylius/AdminOrderCreationPlugin/blob/62485d9c6b7069301cac2b1097825096185e7ac2/src/Provider/CustomerProvider.php#L39-L49 It should use the one from \Sylius\Bundle\ApiBundle\Provider\CustomerProviderInterface so that it can support Sylius Plus properly.
I noticed AdminOrderCreationPlugin has its own CustomerProvider
AdminOrderCreationPlugin/src/Provider/CustomerProvider.php
Lines 39 to 49 in 62485d9
It should use the one from \Sylius\Bundle\ApiBundle\Provider\CustomerProviderInterface so that it can support Sylius Plus properly.