Magento Grunt child theme compilation fails to update merged style files

I’ve set up the Grunt workflow as described here:
https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/css-topics/css_debug.html

My local-themes.js looks like this:

➜  m2 git:(master) ✗ cat dev/tools/grunt/configs/local-themes.js
'use strict';
module.exports = {
    ac_default: {
        area: 'frontend',
        name: 'Ac/Default',
        locale: 'en_US',
        files: [
            'css/source/_extend',
        ],
        dsl: 'less'
    },
    ac_retail: {
        area: 'frontend',
        name: 'Ac/Retail',
        locale: 'en_US',
        files: [
            'css/source/_extend',
        ],
        dsl: 'less'
    },
    ac_wholesale: {
        area: 'frontend',
        name: 'Ac/Wholesale',
        locale: 'en_US',
        files: [
            'css/source/_extend',
        ],
        dsl: 'less'
    }
};

I grunt watch successfully triggers when a less file in my child theme gets changed:

➜  m2 git:(master) ✗ grunt watch:ac_retail -v
Initializing
Command-line options: --verbose, --gruntfile=/data/src/m2/Gruntfile.js

Reading "Gruntfile.js" Gruntfile...OK

Registering Gruntfile tasks.
Initializing config...OK
Loading "Gruntfile.js" tasks...OK
+ black-list-generator, clean-black-list, default, deploy, documentation, legacy-build, mage-minify, prod, refresh, spec, static                          

Running tasks: watch:ac_retail

Loading "grunt-contrib-watch" plugin

Registering "/data/src/m2/node_modules/grunt-contrib-watch/tasks" tasks.
Loading "watch.js" tasks...OK
+ watch

Running "watch:ac_retail" (watch) task
Waiting...
Verifying property watch exists in config...OK
Verifying property watch.ac_retail.files exists in config...OK
Live reload server started on *:35729
Watching pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less for changes.
>> File "pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less" changed.
Initializing
Command-line options: --verbose, --gruntfile=/data/src/m2/Gruntfile.js

Reading "Gruntfile.js" Gruntfile...OK

Registering Gruntfile tasks.
Initializing config...OK
Loading "Gruntfile.js" tasks...OK
+ black-list-generator, clean-black-list, default, deploy, documentation, legacy-build, mage-minify, prod, refresh, spec, static                          

Running tasks: less:ac_retail

Loading "grunt-contrib-less" plugin

Registering "/data/src/m2/node_modules/grunt-contrib-less/tasks" tasks.
Loading "less.js" tasks...OK
+ less

Running "less:ac_retail" (less) task
Verifying property less.ac_retail exists in config...OK
Files: pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less -> pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css                          
Options: banner="", sourceMap, strictImports=false, sourceMapRootpath="/", dumpLineNumbers=false, ieCompat=false                                          
Reading pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less...OK
Writing pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css.map...OK
File pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css.map created.
Writing pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css...OK
File pub/static/frontend/Ac/Retail/en_US/css/source/_extend.css created
>> 1 stylesheet created.
>> 1 sourcemap created.

Done.


Execution Time (2019-01-16 16:13:35 UTC-8)
loading tasks               42ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇ 39%
loading grunt-contrib-less  34ms  ▇▇▇▇▇▇▇▇▇▇ 32%
less:ac_retail              31ms  ▇▇▇▇▇▇▇▇▇ 29%
Total 107ms

Live reloading pub/static/frontend/Ac/Retail/en_US/css/source/_extend.less...
Completed in 0.729s at Wed Jan 16 2019 16:13:35 GMT-0800 (Pacific Standard Time) - Waiting...

Unfortunately the styles-m.css or styles-l.css files are not updated. Therefore when I refresh the page the changes made in the less files are not reflected on the page.

I posted a ticket in the Magento bug tracker but they have not been particularly helpful.

https://github.com/magento/magento2/issues/20356

My question is. Does anyone here uses Grunt to compile less for a setup with multiple child themes?

In my case the intended theme inheritance looks like this:

1. ac_retail inherits from ac_default inherits from luma
2. ac_wholesale inherits from ac_default inherits from luma

From what I can tell the Grunt workflow is the only workflow that is actually suitable for theme development. Server and client side compilation are way too slow to use for development.