It's almost certainly not your problem. DKIM is a cryptographic signature of various parts of the email, which is very commonly broken in all sorts of legitimate ways by mailing lists. This has been a known failing of DKIM & DMARC for quite some time.

However the issue is fixed by the new ARC protocol though, which is supported by most major email providers & in most mailing list software as of like this year. Theoretically, just a matter of time until they update their software & the issue is resolved.