Memory leaks happen when opening and closing emails in new windows (by double clicking) with pinnable add-in with Edge Webview. The memory usage of "Desktop App Web Viewer" process grows to an extent that it might freeze Outlook or reaches add-in memory limit and crashes ultimately, rendering add-ins unusable.
Microsoft has provided a fix for this issue with the August 2020 Microsoft update for Windows users on 1903 and 1909 (see this article from Microsoft for details).
For users on 2004 it was addressed with the September 8, 2020 update (see this article from Microsoft for details, note that this update includes everything in the September 3 preview update that specifically cites addressing the issue with pinned apps.
Although Microsoft has acknowledged they have fixed the issue we are keeping the detail below for historical context.
This was a known Microsoft issue tracked by them on GitHub: https://github.com/OfficeDev/office-js/issues/1069
Users with Microsoft add-ins such as CloudExtend running in Edge Webview. Beginning with Office 365 version 16.0.11629 and Windows 10 version 1903, Office Add-ins running on Office 365 for Windows will use a Microsoft Edge WebView as the runtime. The selection of Edge WebView is entirely decided by Microsoft.
How to reproduce
Open Outlook desktop app and login
Open an email, and open a pinnable add-in (CloudExtend Outlook for NetSuite in this case)
Pin the task pane
Open Task Manager
Double click an email in the inbox, wait for it to open in a new window and wait for the pinnable add-in to load
Close the new window
Repeat Step 4 and 5 multiple times
The memory usage of "Desktop App Web Viewer" keep growing at a significant rate.
Microsoft recommends users install the August 2020 update (see here).
If you are unable to install the update then choose one of the below two options
If keeping the task pane pinned is important to you then periodically open “Task Manager “(Ctrl-Shift-Esc), find ”Desktop App Web Viewer“ processes
right-click → ”End task” (Temporary workaround when freezing happens)
Do not pin the add-in (suggested by Microsoft)
How to 'end task' the Desktop App Web Viewer'