I‘ve gotten the same issue since I’ve given another try to Mail.
I was using Spark before but I wanted to use Mail again as it is a native application and as Airmail wasn’t as good as expected...
I had an Exchange account, 2 gmail accounts and an iCloud account... I‘ve given up with Mail now since Airmail is back on track and is way more reliable and fast than the Mail.... What a shame!
I use an iPad Pro (for my work), iPhone SE and Watch Series 2 with always the last iOS iteration... Airmail shows all emails even on the small screen of the Watch. Which isn’t the case of Mail...
And when the boss of Mail leaves this company for Readdle, what does it say about improvements pertaining to Mail?
I’m afraid nothing!!!!