I found an unusual interaction between this plugin and the
Favourites Menu plugin:
Other plugins (including both Job Spy and MCS) do have their submenus appear correctly, so I'm not sure why Library Codes doesn't. Thankfully I can rename the entries but they're a little bit hard to tell apart when adding them in the first place and it seemed weird enough to report as possible bug.
EDIT: Ok more odd behaviour: I added "Convert ISBN-10..." to the Favourites menu and renamed it. Its icon vanished from the menu and it doesn't actually activate. It's not a big issue because I can use it from the LC menu anyways but it's still strange considering the JS entries work.