Customer profiles cannot hold more than 1 values, even when allowing multiple profiles per user

I have custom API that creates new Customer profiles with different addresses / emails:

$profileStorage->create([
        'type' => 'customer',
        'uid' => $this->currentUser->id(),
        'status'=> 1,
        'address' => $addressData
])
$profile->setDefault(true);
$profile->save();

It works creating those profiles and attaching them to the current user. However, then after whenever I try to edit the user's account fields or just use $user->validate() I get this error:

"customer_profiles: Customer profiles: this field cannot hold more than 1 values."

However Customer Profile Type has both "Allow multiple profiles per user" and "Profiles of this type represent Commerce customer profiles" selected.

This has been the case in Drupal 8 and 9.

UPDATE:

It seems this error actually occurs at $entity->validate(), not on save()

This is the contents of core.entity_form_display.profile.customer.default.yml

uuid: xxx7f0e2-d809-4d2b-9388-af399c98xxxx
langcode: en
status: true
dependencies:
  config:
    - field.field.profile.customer.address
    - field.field.profile.customer.field_email
    - profile.type.customer
  module:
    - address
  enforced:
    module:
      - commerce_order
_core:
  default_config_hash: xxxxVkJkUSPhil9xxxxdTivMqxCUloT_71AHWxxxx
id: profile.customer.default
targetEntityType: profile
bundle: customer
mode: default
content:
  address:
    type: address_default
    weight: 0
    region: content
    settings: {  }
    third_party_settings: {  }
  field_email:
    type: email_default
    weight: 1
    region: content
    settings:
      placeholder: ''
      size: 60
    third_party_settings: {  }
hidden: {  }

and the contents of profile.type.customer.yml

uuid: xxxx4f32-xxxx-xxxx-xxxx-bd10470bxxxx
langcode: en
status: true
dependencies:
  enforced:
    module:
      - commerce_order
third_party_settings:
  commerce_order:
    customer_profile_type: true
_core:
  default_config_hash: xxxxAQDUxxxxwavxxxxlcbJx9xxxxFtCq8l73Axxxx
id: customer
label: Customer
display_label: 'Customer information'
multiple: true
registration: false
roles:
  authenticated: authenticated
  verified_email: verified_email
  verified_cc: verified_cc
allow_revisions: false
new_revision: false

That setting means the customer may have multiple profiles of that type assigned to their uid, not that every profile reference field can itself be used for multiple value references. That said, I'm not aware of a field in Commerce Core called customer_profiles. Is this something custom? You might just need to update its definition to be able to put multiple references in it.