Why does Apple Mail keep disappearing?

That’s how Gmail works. It is neither POP nor IMAP, though it pretends to be either.

It doesn’t have folders, just Labels. A message can be assigned many labels. In an email client, the message would exist in many folders which doesn’t make sense.

Mail, being a standards-compliant email client, will send the IMAP command to move the email message. Gmail then plays its tricks to un-label and re-label the message.

As it is completely non-standard, you should only use Gmail on an email client designed to handle Gmail.

Using app-specific passwords

And the same applies when using Non Native Apple e-mail Clients to access iCloud Mail Services.

Q - " I wish Apple would prioritize getting rid of bugs before adding new features — specially in most used Applications such as Safari, Mail, . . . I use my Mac for personal and business needs. This is inexcusable."

A - Question - Who Owns Gmail ?

A - I do not think Apple owns this Services.

A - Apple Can fix issues that are Directly Related / Owned by Apple like iDevices, Apple Computers, Services, macOS and iOS

One might be better served by communicating with the Owner of this Mail Service

I don't believe these are Apple's problems to correct. It's the developer's responsibility to make their product work with macOS. Not the other way around.

Your can always use your iCloud mail account in Apple's opinion. I happen to like it.

We seem to be moving Side ways from the original question you have posted .

That is ok.

Just how would Apple or, for that matter Microsoft Windows 11 Mail, know just exactly to what mailbox the User ( you ) what the Non Junk mail to be moved ?

Does Google build their software specifically to work with Apple macOS system.

Does Apple macOS build their Software specifically to work on Google Systems.

The answer is, each build their software to work on their versions and types of Systems.

When the User chooses to mix the two or more different types of Platforms - there are often Issue that will and do occur.

Alternatives have been put forth on a volunteer basis, in good faith and in the best interests of the computer.