Case of the Occasional Missing email

This happens once in a while:

I'll see the email on my phone or in Mailmate on the Mac. Then when I come back later to read it, it's "gone." In this case it's a gmail account.

I can't find it in the SpamSieve junk! And it wasn't deleted.

Here's where it should be:
https://www.evernote.com/l/AAF0hJVoVE1LgJcI6j8PKrHb68qjnF_7sec

Then I use the mailmate search box and the message is found, but not in the "all accounts" normal message area.
I can only see it in the "all message subparts" folder.
Here's where I find it:
https://www.evernote.com/l/AAHydEmPdbtKnIAkLseMdZKFO4jSlQ67pks

It's also visible under "sources"

What's going on here?? Should I be using "sources?" I have no idea what the "subparts" folder is, or why this happens!

Thanks!!!!