set ACB's MinimumPrefixLength to 1 char
MODIFY
WalletWasabi.Fluent/Controls/TagsBox.xaml
WalletWasabi.Fluent/Controls/TagsBox.xaml
<c:ConcatenatingWrapPanel.ConcatenatedChildren>
<!-- ReSharper disable once InconsistentNaming -->
<AutoCompleteBox x:Name="PART_AutoCompleteBox"
+ MinimumPrefixLength="1"
IsTextCompletionEnabled="True"
Items="{Binding #PART_Border.TemplatedParent.Suggestions}"/>
</c:ConcatenatingWrapPanel.ConcatenatedChildren>

set ACB's MinimumPrefixLength to 1 char