Not matching small-word followed by word

If the text is: ‘This idea is not helping me at all’ and vocabulary contains a term or synonym of ‘not helping’ it will not match.
If the text is: ‘This idea is helping not me at all’ and vocabulary contains a term or synonym of ‘helping not’ it will match because it’s a word < 3 characters immediately following a non-small word. Is this a bug or a feature to help with performance? If I change the loop to build the $words to check for > 2 then 'not helping' is matched in the above example.

foreach($words_including_small as $key => $word){
if(strlen(trim($word)) > 2){
$words[$key] = $word;
$words_placeholder[] = "'%s'";
}
}

I'm considering adding adding another loop based on checking ($words_including_small[$key - 1]). Is this how you would solve this issue?