I have installed Porter Stemmer v8.x-2.2 module with read me instructions. It doesn't stem few words on Drupal 8 site search
I got this from module maintainer
This is expected behavior of the Porter stemmer, unfortunately. If you examine the list provided on https://tartarus.org/martin/PorterStemmer/output.txt , you'll see that "milder" does not stem down to "mild". In other words, it is not an issue in the Drupal implementation. The recommended solutions include:
(A) adding your own additional implementation of hook_search_preprocess() that manually stems certain words in the manner you want, for the vocabulary that is important on your site
(B) Use the contributed search_api module for your site search, which includes a Porter stemmer option with a UI to add additional keyword stems
It looks good.