I looked into this a bit more and it's clear that a check is done to see if any buttons include submit form actions, and if so, e-signing is disallowed. This may make sense since the form fields, including buttons, are flattened an e-signed document is saved.
I did a test by including a submit form action in a bookmark, and it was successful after e-signing. You can also use a submit form action in a link or use the submitForm JavaScript method in a button, so any of these are workarounds.