I thought I was the only one to have gmails vanish. I've raised the subject on this forum years ago, but now kind of get over it with tricks - one being answering the damn email if allowable, and hoping it will be in Sent.
Their logic is bizarre, though I had wondered if my failing brain just wasn't putting the energy into learning. I have a Yahoo account which has behaved exactly as I expect for decades.
If I read in incoming, it @$#^% off to some place where read and unread join. Just a change in Bold or not. Make it 'Unread' lasts just the once, but doesn't always put it in the unread section. Move does not include an Inbox option. I HAVE to file it away at that point. It used to have, but not anymore.
When I find the missing emails in 'Read', it's often called, me, while I'm looking for the sender's name. No, it's not just when I've answered it.
I hate gmail with a passion, but I'm told I was lucky to get my name with no added numbers, and telling folk the email is always accepted quickly, unlike, @sbcglobal.net which always gets me involved in spelling it out.
I expect that some investment in time would get it under control - of some sort - but in the past, I'd get it usable and then they'd F it up again.