Manual Authenticator mode: Choose where to save the cert and chain files

I’m having an issue with this file naming behavior, although I’m using webroot mode rather than manual. I’m instructing the client to save its resultant files to a specific paths, but the client instead saves the files to names with an added prefix of 0000_ (or such).

Command used: ./letsencrypt-auto certonly --webroot --webroot-path /service/www/htdocs --cert-path /custom/path/to/cert.pem --key-path /custom/path/to/key.pem --fullchain-path /custom/path/to/fullchain.pem --chain-path /custom/path/to/chain.pem --csr /custom/path/to/csr.der

Results desired:
/custom/path/to/cert.pem
/custom/path/to/fullchain.pem
/custom/path/to/chain.pem

Results actually obtained:
/custom/path/to/0000_cert.pem
/custom/path/to/0000_fullchain.pem
/custom/path/to/0000_chain.pem

For now I am working working around this by deducing and renaming the files after the fact, but it would be simpler if the client could be relied on to use the exact file names specified for its result files. (And if a file already exists at that path, I’d rather see an error thrown than have to grep the output for a surprise name, but I could guardrail against that myself.)

Is this a bug, or is this behavior required for some reason? If the latter, is there a way to disable it?