Ok, this technically isn't a bug, but it's definitely an oversight.

The explanation syntax is to be shown when a particular answer is given, irrespective of whether that answer is correct or not.

This makes no sense for a MULTIMULTIPLE question, because:
a) Multiple answers are given, so it doesn't know what explanation to output
b) No single explanation can provide a full outlining of the users mistake

So, for a MULTIMULTIPLE we will instead show the explanations of all incorrect answers given. And this shall be the documented behavior.

Automated response: Explanation syntax makes no sense for MULTIMULTIPLE questions

Because there are multiple answers selected, we can't just give a single hard-coded explanation of why a particular combination of answers are right/wrong. So change the meaning of the syntax for such questions, and document that.

"For [tt]MULTIMULTIPLE[/tt] the explanation text will be shown for any and all incorrect answers (separated by spaces)."

In this case the explanation text is showing a correct answer which wasn't selected, rather than the wrong answer (Forged) which was, removing the explanation text won't change that? Or will it? I mean from a marking perspective.

The explanation functionality may be a little confusing.
I've made it work as would be expected for MULTIMULTIPLE.

Generally speaking though, it provides an explanation associated with the answer GIVEN, whether correct or not. Not an answer NOT GIVEN.
Imagine a quiz that asks about why the sky is blue. There is an answer "Refraction" with an explanation "Did you know that on different planets the sky may be a different colour due to different refractive indexes of different atmospheres." - and another answer "Reflection" with an explanation "If it was reflection then the planet would be a lot darker.".

" In this case the explanation text is showing a correct answer which wasn't selected, rather than the wrong answer (Forged) which was, removing the explanation text won't change that? Or will it? I mean from a marking perspective."

Don't get bogged down in what you were seeing here, it wasn't good code and there's no point trying to understand what that logic was doing as it was stupid. For MULTIMULTIPLE the new code will provide explanations of every incorrect answer, both ticked ones that should not have been, and unticked ones that should have been.

Yes, Velkhanos was not a GIVEN answer but it was a correct answer. Forged was GIVEN and was incorrect, I would expect it to let me know that Forged was incorrect rather than revealing a correct answer. Regardless of the explanation text, I am just wondering if the marking is correct? Is it just the explanation which is confused?

Ahh okay, I missed your 2nd reply before responding. That makes sense. Thanks :)

I fixed marking in #4792. As I made this hot fix after that one and they both touch the same file, the hotfix here includes that fix.

Thanks again, pretty cool feature I haven't really used much. Nice way to use points and almost everyone likes a good quiz.