Postgresql Flexible Server Setting Vnet breaks

Related to customer issue https://github.com/terraform-providers/terraform-provider-azurerm/issues/12053 .

When setting a vnet in Postgresql Flexible Server, it returns "MissingRequiredParameter" error.
It previously worked. Starting from 4th June, it returns error.

PUT /subscriptions/.../resourceGroups/yup-ResourceGroup/providers/Microsoft.DBForPostgreSql/flexibleServers/example-psqlflexibleserver?api-version=2020-02-14-preview
{"location":"eastus2","properties":{"administratorLogin":"psqladminun","administratorLoginPassword":"H@Sh1CoR3!","delegatedSubnetArguments":{"subnetArmResourceId":"/subscriptions/85b3dbca-5974-4067-9669-67a141095a76/resourceGroups/yup-ResourceGroup/providers/Microsoft.Network/virtualNetworks/yup-network/subnets/example-sn"},"storageProfile":{"storageMB":32768},"version":"12"},"sku":{"name":"Standard_D4s_v3","tier":"GeneralPurpose"},"tags":{}}

HTTP/2.0 400 Bad Request
Content-Length: 129
Cache-Control: no-cache
Content-Type: application/json; charset=utf-8
Date: Fri, 04 Jun 2021 02:52:00 GMT
Expires: -1
Pragma: no-cache
Server: Microsoft-HTTPAPI/2.0
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Content-Type-Options: nosniff
X-Ms-Correlation-Request-Id: a87b38eb-a190-c5db-fcf1-46cae3706a4a
X-Ms-Ratelimit-Remaining-Subscription-Writes: 1198
X-Ms-Request-Id: 6fdcfd6e-58ab-45c5-aaf0-bbfbe409fe96
X-Ms-Routing-Request-Id: SOUTHEASTASIA:20210604T025200Z:f5b1996d-e4ab-4895-a953-a5a90d696030

{"error":{"code":"MissingRequiredParameter","message":"Parameter '' must be specified. This parameter cannot be NULL or empty."}}

Start by reproducing the documented PUT request for PostgreSQL Flexible Server with the delegated subnet arguments shown in the report, then compare the 2020-02-14-preview specification with the failing request. Confirm which required parameter is absent or mismatched and add a regression check for the corrected request; the issue supplies no file or test path.

Written by the indexing model from the issue text.