And with a changelog here: https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.12/P3
Will break signatures with inline images, but not quite like Zimbra puts it ("Inline attachment shows as an attachment in compose window.")
What's worse is that *every time* the message is saved to Drafts, the inline image in the signature is added, again, as an attachment. If you have 30-60 seconds between auto-save-drafts, you can easily end up with 5-15 attachments, depending on how fast you type and how much you type. This error is quite easy to reproduce by simply clicking the "Save draft" button.
We've reported this to Zimbra. The response so far isn't overwhelming.

-joho