This is *extremely* untested on 5.12.x (and, much more generally, on > 5.9.x). So, no idea. If you're familiar with fontconfig, you can check the generated shards for obvious mistakes and double-check the mappings via fc-match.
While I haven't bothered to check because it'd be a waste of time without a device to test it on, I would expect this to be broken on FW where Amazon added support for custom fonts, so, it's not terribly surprising.
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FreeType Overrides are (safely) ignored on FW > 5.8.x (and/or earlier, not that it matters here), so that's irrelevant

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That just means Code2000 is at the bottom of the matching list (as it should, it's got decent coverage, but it's fugly as hell, and has weird metrics). Even with a fallback override set.