As someone else pointed out this is a bad translation of English. In English it would be said that, "It's a good thing I left Miguel!" I don't buy the British translation that was proposed as I have literally never heard that.
And as someone else already answered, this phrase is perfectly normal British English. Doesn't mean it doesn't exist just because you haven't heard of it. "It's a good thing..." doesn't role off my tongue so I doubt I'd ever say it, although I'd absolutely understand the meaning if someone said it to me.
Perhaps you could politely ask the Kwiziq team to add a US English translation, as they do for other British English terms like 'holiday' and 'rubbish? But please don't ask that a UK-based team move away from standard British English phrases.
Have you considered that there are many Spanish learning resources in US English (e.g. Duolingo) that don't work well for me and others because they don't recognise non-US English words/phrasing? What you've experienced here is what we find everywhere else!
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