I was calling bullshit on you with your "abolishing support for mapper 23" comment, as that seems incredibly unlikely, and a glance at the NEStopia support indicates it still supports mapper 23.
But honestly, I don't care. Blame the emu if you want. Whatever.
Let me start out by saying I owe you a big fat apology. Now that I got some more sleep and am less cranky, I looked at the source a bit closer and it does in fact look like you are mostly right.
The thing about mapper 23 is that it represents multiple board types. In an effort to differentiate which one to emulate, NEStopia is searching for the board name in the database....
... which doesn't make sense, because if you're using the database, then why do you need the mapper number?
... and if you're relying on the mapper number then the file obviously wasn't in the database...
I don't know if this was an oversight, a bug, or just shitty design, but yeah. It's fucked. You're totally right.
What it isn't, though, is "accuracy".