I phone 2020 SE losing g mail when put into a folder

When you look at the settings for the Gmail account by going to Settings>Mail>Accounts, if you tap on the Gmail account, does it show IMAP on the top of the next page? I'm don't have an idea why the mail would disappear that way. Although I do know that when I am looking at Gmail in the web access, how it deals with folders and things can be a little strange. In the Gmail app there is a command to see all email, which then includes the Inbox as well as how they separate it as Promotions, etc. I just wonder if that is what maybe changing things, even though you created your own folder. Try going to the web access and see if you can find the mail that way. It may still be in the account, just the way that things get displayed is more difficult to understand. You may also want to consider using the Gmail app, since it contains all of the features of being designed by Google and may work out better for you. Also, with the app, you can get your mail Pushed to you, where that isn't possible with the native Mail app on the iPhone.

Are you using the native Mail app on the iPhone, or are you using the Gmail app? If the Gmail app, then you would need to contact Google Support for assistance.