Composer package is updated, but core still asks to update
CREDIT TO @CGMonroe 

I had started since early stages of Drupal 8, so my themes folder was in 

sites/default/theme/bootstrap
sites/default/theme/bootstrap_subtheme


but in composer.json, it updates in directory:

"installer-paths": {
  ...
  "themes/contrib/{$name}": ["type:drupal-theme"],


so what I needed to do was, move my /bootstrap_subtheme from sites/default/theme to theme/
you should end up with:

theme/contrib/bootstrap
theme/custom/bootstrap_subtheme


and here is the correct way of configuring a theme/subtheme: https://www.drupal.org/docs/8/theming-drupal-8/creating-a-drupal-8-sub-theme-or-sub-theme-of-sub-theme