After applying the 8.1.3 update, you may observe new Client Accounts being created upon order with empty profile details.

A user who is not signed in, and is already associated with more than 1 account places an order, then at checkout logs in and opts to create a new client account.

A hotfix has been created for 8.1.3 only to improve the inheritance of the parent theme property:

https://whmcs.community/files/file/164-core-16363-prevent-account-creation-during-checkout-for-user-with-multiple-accounts-unless-requested/