The old method didn't work with some third party context menu handlers that need to be run via the shell.
The new method does work, but only when they're added to the actual file type that Windows believes the extension to be, because the Windows shell apparently gets confused when asked to run something otherwise.
When they're added to one file type (mpc-be64.mkv) but that is then taken over by another (mplayer64.exe, I think it was in this case) is where you run into problems. But those menu items shouldn't really even be shown, so if we fix this it'd just be to hide the broken menu items entirely so we're at parity with File Explorer.
(And we could potentially make the file type editor edit the overriding file type rather than the overridden one, but that's quite complicated. We could also just remove the file type editor, which is what Microsoft did to theirs years ago when they made things too complex for themselves... But it's useful and we don't want to do that, even if it's imperfect. I don't think the file type editor is where those menu items came from anyway.)
You only need to add the menu item once, to the Movies file type group.
To remove all the non-working menu items at once, configure MPC-BE and tell it not to add its menu items by turning off these two checkboxes:
(Those are what we're looking at here; none of them are self-created in Opus 11, they are what MPC-BE creates when these checkboxes are turned on.)
If the checkboxes are greyed out, you need to click the Modify button shown below, and accept the UAC prompt, so that MPC-BE has access to modify the registry: