Odd kmtrigger:// behaviour

I'm trying to invoke a keyboard maestro macro from an Anki card (yeah long story).

Anki let's you include hyperlinks on it's cards, and recognises url schemes. I can invoke marginnote:// drafts:// etc and they work fine.

However putting in kmtrigger:// doesn't work- Anki just says "loading" and the request never gets to Keyboard Maestro. Running the same kmtrigger url from elsewhere (eg drafts, from a browser etc) works fine.

My markup looks as follows- the first one works, the second one does nothing.

<a href="drafts://x-callback-url/runAction?text={{uid}}&action=RoamCatcher">*</a>

<a href="kmtrigger://macro=LoadRoamBlock&value={{uid}}">*</a>

I've got a workaround which is to route the request through drafts- the drafts action above just proxies the request through to Keyboard Maestro (using the same url as above).

However I can't work out why one url scheme works and one doesn't- is there something special about how Keyboard Maestro url schemes are registered? It could be an Anki issue but I think they are just rendering html.