There's a menu bar for the entire email chain, and an "Actions" button for each mail message. Report Spam is only available in the former, and Report Phishing is only available in the latter.
I agree, it's annoying! Maybe someone at Fastmail will see this.
Guess they really live up to their promise
Maybe auto-suggestions as you type into certain fields? You’re typing into the location field of a new event and received a hotel booking receipt email in the last 2 hours, perhaps the address of that hotel could be a smart auto-suggestion? Things like that.
With a brand new user account, I was not at all able to reproduce your issue. I closed the browser and cleared cache and all that, but I did not see an entire page refresh at any point.
Perhaps there's some weird caching issue with your browser, or maybe you've got a browser extension misbehaving? Maybe I'm just lucky? Idk, but hopefully this helps.