So Gmail forums say this issue lies with the various email clients. In this case, I will discuss Outlook 2019 (this has always been an issue even with other versions).
Within Gmail, some email tends to "hide" and is only available within Gmail's self-created folder called ALL MAIL. Despite ALL MAIL being shown and enabled in Gmail settings, it doesn't carry over to ANY email client. For discussion purposes, I want to clarify and emphasize ALL MAIL consists of only ONE email account. It's not a folder consisting of various other email accounts.
Gmail support team are playing the blame game.
Anyway, I thought I would post here to see if others have experienced the same (assuming if they care) and what was their workaround?
