Using Groupy on Win11, I create an "Application Grouping" for notepad (..two notepad windows grouped together, right click, "Automatically group all 'Notepad.exe' windows together"). In Settings, "Application Grouping," an entry gets created that looks like this: "Microsoft.WindwosNotepad_8wekyb3d8bbwe!App | Automatically group". However, notepad does not automatically group. It will group if I drag and hover a newly create window, or if I click the "+" to add to the group. Note: Another entry in "Application Grouping" works great; it's settings look like this: "explorer.exe | Never automatically group". Since that enter is a 'never group,' I created a always group for windows (MS Word). It too worked great.
Solution: to the registry. Warning: be careful, make a backup, don't make a mistake, don't fat finger and accidently delete HKCR, do at your own risk, "Danger, danger Will Robinson", and so forth.
On the right side, locate the REG_SZ entry named "Microsoft.WindowsNotepad_8wekyb3d8bbwe!App". To rename the key, press F2 (or right click and select rename).
Enter the new name: notepad.exe
Press enter ONE time. (If you press it twice, you're now editing the value, which should be a: 1
(Value of 1 means "Automatically group"; Value of 2 means "Never automatically group"
For those SysInternals Suite users, here are the three good lines for you. Everyone else, just two good lines.
reg delete HKCU\Software\Stardock\Groupy\Groupy.ini\Groupy\AutoGrouping /v Microsoft.WindowsNotepad_8wekyb3d8bbwe!App /f
reg add HKCU\Software\Stardock\Groupy\Groupy.ini\Groupy\AutoGrouping /v notepad.exe /t REG_SZ /d 1 /f