Bud 16.4.3 update conflicts with postcss-mixins package

Updating Bud from 6.12.3 to 6.14.3 causes a conflict with postcss-mixins. npm run build completes successfully after the update, but mixins are not processed and the output css still contains the @define-mixin and @mixin declarations from the pre-processed css files. This causes the affected styles to not be loaded by the browser. Any thoughts on how to fix this?

Example source CSS

@define-mixin heading {
    font-family: 'Roboto Slab', serif;
    font-weight: 400;
}

h1, .h1 {
    @mixin heading;
    font-size: clamp(2.25rem, 1.75vw + 1.594rem, 3.125rem);
    line-height: clamp(2.7rem, 1.651vw + 2.273rem, 3.594rem);
    margin-bottom: 1.25rem;
    margin-top: 1.25rem;
}

Bud 6.12.3 output

.h1, h1 {
    font-family: Roboto Slab,serif;
    font-weight: 400;
    margin-bottom: 1.25rem;
    margin-top: 1.25rem;
}

.h1, h1 {
    font-size: clamp(2.25rem,1.75vw + 1.594rem,3.125rem);
    line-height: clamp(2.588rem,1.7vw + 1.95rem,3.438rem);
}

Bud 6.14.3 output

.h1, h1 {
    @mixin heading: ;
    font-size: clamp(2.25rem,1.75vw + 1.594rem,3.125rem);
    line-height: clamp(2.7rem,1.651vw + 2.273rem,3.594rem);
    margin-bottom: 1.25rem;
    margin-top: 1.25rem;
}

package.json

"scripts": {
        "dev": "bud dev --browser",
        "build": "bud build"
    },
"type": "module",
"dependencies": {
    "@fortawesome/fontawesome-svg-core": "^6.4.0",
    "@fortawesome/free-brands-svg-icons": "^6.4.0",
    "@fortawesome/free-regular-svg-icons": "^6.4.0",
    "@fortawesome/free-solid-svg-icons": "^6.4.0",
    "simple-datatables": "^7.2.0"
},
"devDependencies": {
    "@roots/bud": "^6.12.3",
    "@roots/bud-postcss": "^6.12.3",
    "@roots/bud-tailwindcss": "^6.12.3",
    "postcss-extend-rule": "^4.0.0",
    "postcss-mixins": "^9.0.4"
}

bud.config.js

/**
 * Build configuration for bud.js
 * @url https://bud.js.org/docs
 * @param {import('@roots/bud').Bud} app
 */
export default async app => {
    /**
     * The bud.js instance
     */
    app

    /**
     * Paths
     */
    .setPath({
        '@src': `assets`,
        '@dist': 'public',
        '@ssl': '/users/sanered/.config/valet/Certificates'
    })

    /**
     * Application entrypoints
     * Paths are expressed relative to the `@src` directory
     */
    .entry({
        blockSupports: ['scripts/blockSupports'],
        datatables: ['scripts/datatables', 'styles/datatables'],
        editor: ['scripts/editor', 'styles/editor'],
        editorVariables: ['styles/editor-variables'],
        global: [`scripts/global`, 'styles/global'],
        lightgallery: ['scripts/lightgallery'],
        wp: ['scripts/wp', 'styles/wp'],
        wpResetEditorStyles: ['styles/wp-reset-editor-styles']
    })

    /**
     * Copy static assets from @src to @dist
     */
    .assets(['images', 'fonts', 'sections'])

    /**
     * Create sourcemaps
     */
    .devtool()

    /**
     * Dev server setup
     */
    .proxy(app.env.get('WP_HOME'))
    .serve({
        host: 'lib.jmu.test',
        cert: app.path('@ssl/lib.jmu.test.crt'),
        key: app.path('@ssl/lib.jmu.test.key')
    })
    .watch(['templates', 'app/composers', 'app/config'])

    /**
     * PostCSS
     * @url https://bud.js.org/extensions/bud-postcss
     */
    .postcss
    .setPlugin('postcss-extend-rule')
    .setPlugin('postcss-mixins')
    .setPluginOptions('env', {stage: 1})
}

Details

MacBook Pro: Ventura 13.4.1
Node 16.20.0
Valet
WP 6.2.2

Do you need to add your postcss plugins to .use()?

You can see it being done here in bud-tailwindcss.

Yes, that did it. Thanks!