The reason I ask is because I do all my Bootstrap SASS editing in Pinegrow (also in the Pinegrow editor) with Sublime text 3 next to it opened and noticed it only works when at least one HTML or PHP page from the Bootstrap 4 project is openend in Pinegrow! I read that you have all files closed? That can be the reason it doesn’t work!
Dit you try the same procedure with a page from the project openend?