Apple Mail won't save signatures on exit

I have searched a bunch of forums and found a bunch of answers, all of which did not actually answer my question on HOW to get Apple Mail to save signatures upon exit. After exhaustive (weeks worth) research, I have found the answer. I hope this helps you too.

I had been using iCloud to manage most everything on my laptop (mid 2014 MBP 16gb, 512, i7 running High Sierra), including mail. All of my email addresses were either gmail addresses or were hosted by gmail. As soon as I stopped iCloud from trying to sync my mail, restarted my machine, voila! Signatures would save on exit of Apple Mail.

It bears noting that I had to turn iCloud sync off in two (2) places in preferences in order for this to work. First in iCloud main preferences screen...

then in the iCloud Drive folder...

I think the key here is that Gmail is managing email, so iCloud doesn't need to. IF you have iCloud try to manage it, it causes a conflict that causes this issue, or it did for me.

Also as a side note, all of my gmail accounts are set up as imap so I can have them sync across devices easily.

Again, I hope this helps.