Well, it can't fold them and have them fit in the 78 character limit without adding whitespace that isn't in the original headers (unless there's a more subtle bug :)

The email package has the possibility of having special behavior based on the name of the header, so if DKIM headers have special rules, there is the possibility of implementing those special rules.  Basically, you can implement a parser that recognizes dkim headers and represents what parts can legally be folded using the resulting parse tree.  So it may be possible to fix this without changing the refold_source default.  It is also possible to specify that encoded words may not be used in a given header (that's a simple toggle), which may be all that is needed here.