This also happened to me. A very important email was put under the promotions. Usually I just kept the gmail tab open and didn't check my phone. Fortunately for me, I noticed this mail on my phone when I was clearing the unread status.

At the same time, all sorts of ads fall into my inbox repeatedly, even I deliberately mark them as spam or junk.

( Being cautious, I just checked the promotions again. And there is another important email lying. WTF!)