@wguida
A couple of thoughts come to mind.
It's possible that the Canvas Live API is mangling the request somehow. It's designed for exploration, not really for serious work. Once you figure out how a request works, then you can use a REST API client (a lot of people like Postman) or your favorite programming language (all the popular languages support requests). If the request works there but not through the Live API, then I would suspect it's something the Live API is doing. I haven't used it in years because I moved past it.
A second thought is that there is an error in your HTML that is causing it to get stripped. The thing that comes to mind is that you're missing an opening or closing quotation symbol or tag. If the HTML is improperly formed, it might be getting sanitized, causing the anchor element to be removed or non-functional.
I'm sure there are other possibilities, but those are the two that come to mind. Without knowing exactly what you're sending, it's hard to tell. I would probably start with the HTML code itself and make sure it's valid.