There are a lot of questions on this topic. I have been through them and not found an answer, though located the specific issue.

The following watch will only execute et-js the first time that is modified:

gulp.watch( './js/et.js' ).on( 'change', gulp.series( 'et-js' ) );

It will only run every time the file(s) are modified if it has more than one file suplpied, e.g.

gulp.watch( ['./js/et.js', './js/tu.js'] ).on( 'change', gulp.series( 'et-js' ) );

This doesn't run on every modification either:

gulp.watch( ['./js/et.js'] ).on( 'change', gulp.series( 'et-js' ) );

Any ideas what the issue is?

Gulp version:

CLI version: 2.3.0 Local version: 4.0.0