Just posting this to fellow JetBrain IDE users.
I have spent a considerable amount of time troubleshooting why changes to my SASS-files were picked up. I develop in PHPStorm on Windows 10 with a CMD in the background running "sencha app watch".
All of a sudden I noticed that changes to the code worked, but changes to the styling did not happen.

I made the standard "sencha app clean" and "sencha app refresh" and then resumed "sencha app watch" - still no go! Out of desperation I removed the whole "build" directory from the project and started "sencha app watch" again. Viola - working again.

When it broke again I took my time to look into the problem. From sencha cmd I saw some errors like
TypeError: undefined is not an object (evaluating 'scanner.peek().value')
when compiling the scss. After that it didn't matter what changes I made to my SASS-files - they never bit. I cleaned out build/temp/development/proc8ui/sass and all well again.

Somehow the bad files got there and then I realized that it may be me creating them by switching between PHPStorm and the docs open in a browser! When switching I often leave a comma missing or simply producing code with syntax error.
Checking the settings, my PHPStorm was configured to "Save files on deactivation" meaning that all files are saved when I switch to the browser and changes picked up by "sencha app watch", producing bad files if there are any issues. After disabling this setting all is well.

I think that "sencha app clean" should really clean the temporary directories and files left behind by the watch builds to produce a clean environment for "sencha app watch"