It is impossible to enter Page mode when framework.assets is set to json_manifest_path. This is core Symfony feature since 3.3.

More information about this feature: https://symfony.com/blog/new-in-symfony-3-3-manifest-based-asset-versioning

Steps to reproduce:
1) Create clean eZ Platform EE v2 instance,
2) Install symfony/webpack-encore*
3) Create webpack.config.js*
4) Set configuration key framework.assets to json_manifest_path: '%kernel.project_dir%/web/build/manifest.json' (use path which you defined in webpack.config.js).
5) Login to admin interface
6) Go to the Page tab

You should see a warning:

Warning: preg_replace(): Unknown modifier '/'

Thanks to the customer you can see the whole exception page as a screenshot in the attachment.

symfony/webpack-encore installation:
1) from the project root directory execute:

npm install @symfony/webpack-encore --save-dev

2) create webpack.config.js file in the project root directory, with content:

// webpack.config.js
var Encore = require('@symfony/webpack-encore');

Encore
// the project directory where all compiled assets will be stored
    .setOutputPath('web/build/')

    // the public path used by the web server to access the previous directory
    .setPublicPath('/build')

    // will create web/build/app.js and web/build/app.css
    .addEntry('app', './web/js/7b50630.js')

    .enableSourceMaps(!Encore.isProduction())

    // empty the outputPath dir before each build
    .cleanupOutputBeforeBuild()

// create hashed filenames (e.g. app.abc123.css)
// .enableVersioning()
;

// export the final configuration
module.exports = Encore.getWebpackConfig();

Note: You can safely choose other file in .addEntry('app', './web/js/7b50630.js') actually it doesn't matter in the context of this bug.
3) Run webpack:

./node_modules/.bin/encore dev --context=path/to/your/project/root/dir

Note: If you want to run webpack for prod then change dev to production.

more: http://symfony.com/doc/3.4/frontend/encore/installation.html & http://symfony.com/doc/3.4/frontend/encore/simple-example.html