One thing that I had to do as well is listen to 'end' event to call done(). Example: From https://www.liquidlight.co.uk/blog/article/how-do-i-update-to-gulp-4/
gulp.task('do-stuff', gulp.series('other-dep', function (done) { return gulp.src(someFiles) .pipe(doStuffToThem()) .on('end', function () { done(); }); }));