In this case, 'smartcase' does exactly what it should do: your input has at least one uppercase character which makes completion default to case-sensitive matches.
This is documented in :help 'smartcase' and :help 'ignorecase'.
There are several ways to work around this:
Input lowercase characters only:
:avantec<Tab>Use the correct case:
:AvanteC<Tab>Disable
smartcase::set nosmartcase :Avantec<Tab>