Key serialization formats can be - uh - the source of "interesting" issues. It appears the whole internet technically uses DKIM the wrong way, but it's more or less the fault of the standard.
DKIM uses public keys in DNS, usually RSA, but how are they encoded? There are two common RSA public key formats, SPKI and PKCS#1.
The DKIM spec RFC 6376 says this should be an RSAPublicKey and references RFC 3447, which is PKCS #1. So it's PKCS #1, right?
Well... there's an "INFORMATIVE" part of the RFC that lists openssl commands to encode a key, with an example. And that's... the openssl command to generate SPKI. The example shown is also an SPKI key.

The Internet has voted with its feet and everyone uses SPKI. From previous research, I had a collection of ~35k DKIM keys, and there are zero PKCS#1 keys in there.

This appears to be known and is mentioned in the errata.

It's quite an unfortunate situation. Technically, everyone's doing it wrong. However, if you would happen to be so brave to try to do it right, you'll probably just run into problems. While I haven't tested it, my best guess is that you will almost certianly find some receivers accepting PKCS#1 and others not. (Many crypto library APIs autodetect the format, but given *noone* is using PKCS#1, I'm sure there will be ones only accepting SPKI.)