Solved it. I had to add that "webflo/drupal-core-require-dev" to update command to be updates as well, because without that core couldn't be updated:
composer update drupal/core "webflo/drupal-core-require-dev" --with-dependencies
Saw that in "known issues" section. :/
It's really strange - what's the purpose of --with-dependencies when I have to add dependencies manually after all...