I think you must be describing using the iOS 11.3 beta 1 or 2, not the Fastmail app version (which is currently 1.2.7). I would expect bugs from Apple on beta releases. The iOS 11.3 beta 1 (released Jan 24, 2018) had several nasty bugs, causing many testers to revert to the 11.2.5 public normal release. iOS 11.3 beta 2 (released Feb 6, 2018) has fewer reported bugs.
I recommend using the official Apple iOS release if you want to have the Fastmail app work correctly.
Bill
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