(Blowing the dust off this old one, as all similar google community threads are locked)

Having the same issue here, with the following setup:

Private Gmail with external domains that just forward to this gmail account using cloudflare. I like to send myself mails for reasons.

Emails to all these external accounts won't show up in inbox, but in sent. Pretty much like https://stackoverflow.com/a/52534520/635876 said - gmail recognizes the mail coming from and going to the same account and is not showing it in inbox.

I was able to get them into my inbox by adding an explicit filter for those mail addresses, setting to 'not spam, mark important, categorize as general'.

They show up as read, but that's fine for me as my inbox is kinda' my todo list.