Too bad you are having a poor experience. I don't have an Android device, so I can't give you any direct experience on that platform. The Fastmail iOS app works great for me, as well as the website (with Windows PC browsers) and email clients (iOS Mail, Thunderbird, and Outlook).
I suggest that you give Fastmail direct input (via support requests) about their Android app, since they seem to be interested in improving it.
Bill
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