iCloud emails instantly disappeared

Yeah I’ve been using my iCloud account for 10 years and this is the first time I’m experiencing this. I’m not sure “where” it happens honestly, but both times I got the notification on my iPhone’s lock screen, but no email at all on the Mac. I even went to iCloud.com and the email was already gone, even before I tapped on the notification on the iPhone – and it makes sense, because as soon as I tapped on the notification and opened the email on the iPhone, the message opened for 2-3 seconds and then closed by itself, clearly because it was already gone from the server.

It happened again twice. One thing I've realized is that I've installed the iPadOS 14 Public Beta on my iPad, so I wonder if that's creating the problem (being a beta). I've downgraded to iPadOS 13, hopefully it doesn't happen anymore.

It seems like the issue is Apple Mail on macOS Catalina (I'm on 10.15.6).

A daily email that I receive every day from USPS has constantly disappeared every single day for the past week. I receive it at around 7am every day. I see the iOS notification, but no notification on the Mac Apple Mail app, and as soon as I open it on iOS, it closes and disappears after 3 seconds. It's not in the Trash or SPAM.

Now that I've stopped using Apple Mail and started using Spark instead, I received that email today, and for the first time in more than a week it didn't disappear.

From what I can see, Apple Mail on macOS Catalina is the problem. Not sure why, I don't have settings against SPAM and I had remove the option to automatically delete emails after a certain amount of time.

I like Apple Mail, but now I'll wait until macOS Big Sur to see if the problem has been solved. Maybe my findings can be helpful to somebody else.