Thanks for hopping in this conversation, @MG-VI. I appreciate you following the steps shared by my colleague. 

Yes, I got you some workaround to successfully save the subject name of the emails. You can include the word Estimate in your subject such as Order Acknowledgment/Estimate, then click Done to save your work. 

Another option is to leave it as Order Acknowledgment, then close it by clicking the X button next to Send feedback. After that, select Save and Exit when prompted. See screenshots below: 

That will save the changes you've made on the subject of the emails. 

For future reference, check out this article: Import custom form styles for invoices or estimates. It helps you learn about how to design your sales form to make it more personalized and import it in QuickBooks. 

I'm always around here to help in case you need more help. Take care.