Sync networking configuration for Azure App Service hosting plan
It might happen that although you integrated your app with an Azure Virtual Network, you can't establish connection to Managed Instance. One thing you can try is to refresh networking configuration for your service plan.
Sync network configuration for App Service hosting plan
To do that, follow these steps:
Go to your web apps App Service plan.
Click Networking and then click Click here to Manage.
Select your VNet and click Sync Network.
Wait until the sync is done.
You are now ready to try to re-establish your connection to your Managed Instance.