Hi Camner,
this is strange. Usually the Fastmail iOS app (as well as the Fastmail web client) only suggests contacts from your (Fastmail) address book, I just verified this on my phone. Are you sure the suggested addresses aren’t in your FM contacts? Do you have "Auto-save contacts" turned off in Settings → Preferences → Advanced Settings, Section "Compose & Reply"?
Cheers,
Michael
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