Apparently, the board used by Karnov uses hardwired mirroring (it never writes to the MMC3's mirroring register). Nestopia's ROM database marked it as using 4-screen mirroring (as far as I can guess), while Nintendulator was initializing it to vertical mirroring.
The ROM itself is marked as using vertical mirroring, while it should be set to horizontal.

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Quietust
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