We got bunch of unpaid invoices. When post one payment for several invoices, system automatically posting to wrong invoices and I have to go back and uncheck them all

You will need to turn off “Automatically apply bill payments” feature in new QuickBooks.
To do this task:
From the homepage screen,

• Select the Company “Gear” (upper-right-corner)
• Select Company Settings under Settings heading
• Select “Advanced” under Settings
• Select “Automation” under Advanced heading
• Select and turn off “Automatically apply bill payments” feature. See attached screenshot photo for reference.
• Save