This is odd. On my Windows 10 laptop here, I unpinned Emacs (after having fussed with it over the last couple weeks), verified there was no lnk file left.
Started emacs with runemacs. Lnk file was created and with lnk_parser I could see the AppUserModelID of GNU.Emacs had been set in the shortcut.
Starting another instance of Emacs resulted in two icons correctly grouped in the taskbar.
I know when I first tried pinning Emacs on Win10 a couple weeks ago to test it for David, it did *not* do that. I'm suspect some other registry entry is still there even after unpinning it. (Like the AppPaths registry entry.) And that is helping Windows determine the AppUserModelID. If we knew what doing that and we could do that.