When there are completion suggestions such as println! (in Rust macros end in !) or nil? (in Ruby methods that return a boolean end in ? by convention), triggering the Autocomplete action (Tab by default) will not cycle through them. CycleAutocompleteBack (Shift-Tab by default) works fine though.

Normally micro will never offer suggestions that have non-word characters so this problem only occurs when plugins use micro's auto-completion feature. It has been annoying me when using LSP completions.