Welcome to the Community forum, @Orchidist. I appreciate you looping in this thread about having the same issue on a different QuickBooks product. We hear your sentiments about the resolutions and updates, so I'll provide some news for you.

You are correct that INV-90451 has been resolved. It was related to QuickBooks Desktop 2023 version which used to crash when opening the file. This has been fixed in the latest release on R6_22 on the Product Information window (press F2).

I understand that you are facing issues with QuickBooks Desktop Pro 2021 crashing when you close it. After checking my resources, there's a current report (INV-93965) regarding this problem. Therefore, I recommend contacting our Support Team again. They will add you to the list of affected users to ensure you'll receive prompt solutions as soon as they are available.

Here's how:

1. Go to the Help icon, then select QuickBooks Desktop Help.
2. Press Contact Us.
3. Enter a description of your subscription concern, then click Continue.

Please check out this article for our support hours:  Contact QuickBooks Desktop Support. It is to ensure we cater to your needs on time.

Scan this guide to learn the potential causes why the program is frozen as well as their troubleshooting: Fix issues when QuickBooks has stopped working.

With this information, you should be able to clarify to our representatives that you're part of a new investigation specifically for Pro 2021. Rest assured that our Team is working hard to address this quickly. If there's anything else I can help you with, add them here so I can help you more.