I have experienced last month, our DKIM was neutral on Gmail, but every other domains the DKIM passed.
After a couple hours of test, we figured out that the body HTML of the message had some lines without spaces (12k caracters in a row) was interfering on DKIM result.
I didn't know the body of HTML cound interfere on it (and i still don't understand why it happens). But after the code were indented ( with this 12k caracters line broke in many other lines) the DKIM was passing again.
It's not the same problem, but you can check it. Just to be sure.