So I think I figured it out. The file in question must have been created with just the --sign option and not both --sign --encrypt. Based on looking at the file relative to another that was both signed and encrypted, you can see they both have signatures but the file created with only --sign does not have a :encrypted data packet and only has the literal data packet:
(10:35) gpg > gpg --list-packets test2.txt.enc.asc :pubkey enc packet: version 3, algo 1, keyid data: [2048 bits] You need a passphrase to unlock the secret key for user: "John Doe <jdoe@email.com>" 2048-bit RSA key, ID ********, created 2015-02-28 :encrypted data packet: length: 405 mdc_method: 2 gpg: encrypted with <a key> :compressed packet: algo=2 :onepass_sig packet: keyid version 3, sigclass 0x00, digest 8, pubkey 1, last=1 :literal data packet: mode b (62), created 1463246548, name="test2.txt", raw data: 32 bytes :signature packet: algo 1, keyid version 4, created 1463246548, md5len 0, sigclass 0x00 digest algo 8, begin of digest 35 d8 hashed subpkt 2 len 4 (sig created 2016-05-14) subpkt 16 len 8 (issuer key ID) data: [2045 bits] Versus:
gpg > gpg --list-packets test2.txt.asc :compressed packet: algo=1 :onepass_sig packet: keyid version 3, sigclass 0x00, digest 2, pubkey 1, last=1 :literal data packet: mode b (62), created 1463246421, name="test2.txt", raw data: 32 bytes :signature packet: algo 1, keyid version 4, created 1463246421, md5len 0, sigclass 0x00 digest algo 2, begin of digest b9 a7 hashed subpkt 2 len 4 (sig created 2016-05-14) subpkt 16 len 8 (issuer key ID) data: [2047 bits]