I don't know what is happening with Gmail but lately Gmail's aggressive spam filtering created a huge problem for me.
It was a reply I sent to an email yet Gmail somehow figured out it was spam and the other person never got it (which I learned after 1 week)
Two weeks before they sent the property tax notice (from the govt) in spam and had I missed that email there would have been a penalty.
This is a very old Gmail address almost 10 years or more so I don't know what's happening but I've stopped trusting Gmail altogether after this.
This spam thing is such a black box, so don't be like me and check your spam folder just like you check your inbox if you're on Gmail.
P.S. Also looking for a good alternative to gmail if anybody has some suggestions. Right now I've just created a filter to send all email to inbox.
100% Fastmail. I've been on them for eight years now, after switching from a decade of Gmail. I've had zero complaints.
Switching from Gmail isn't even that bad - you can keep your Google account using your new email address as the ID, so that you can still use Docs and Drive and such. I wrote a short guide here: https://www.justus.ws/tech/how-to-ditch-gmail/