Upon further testing, it seems a similar sort of behavior occurs when the . prefix is used to "silence" the "construct command", except that it, of course, simply runs the first alias in quiet mode, (i.e. with $show == $false), if it occurs on the same logical line. Unlike the ! prefix buggy behavior, however, this will occur even if the alias is used as an identifier - given:
Code
alias -l quiet_alias {
  echo -ga silenceable alias invoked as $iif( $isid , an identifier , a command ) with $!show == $show
  echo -gaq this message is only visible in verbose mode
}

alias pfxtest_5 {
  .if ( $true ) {
    quiet_alias
  }
  .if ( $true ) {
    $quiet_alias
  }
}

alias pfxtest_6 {
  .if ( $true ) { quiet_alias }
  .if ( $true ) { $quiet_alias }
}
.../pfxtest_5 will print both messages within /quiet_alias in both cases, while /pfxtest_6 will only print the first (non-silenceable) message in both cases.