13.0.0 Beta 3: Wrong paths in App CSS files with multiple app_paths #7534

Steps to reproduce

1. Open the Polls App without any existing polls (the same problem exists for example with the Mail App)

Expected behaviour

In the middle of the screen, there is the icon of the polls app.

Actual behaviour

There is no icon. There is no problem with the icon in the navigation bar on the top.

After looking at the CSS code, there is

mask: url('../../apps/apps2/polls/css/../img/app-logo-polls.svg') no-repeat 50% 50%;

It should be

mask: url('../../apps2/polls/css/../img/app-logo-polls.svg') no-repeat 50% 50%;

Server configuration

Multiple app paths are set in the config.php:

  'apps_paths' =>
  array (
    0 =>
    array (
      'path' => '/.../apps3',
      'url' => '/apps3',
      'writable' => false,
    ),
    1 =>
    array (
      'path' => '/.../apps2',
      'url' => '/apps2',
      'writable' => true,
    ),
    2 =>
    array (
      'path' => '/.../nextcloud/apps',
      'url' => '/apps',
      'writable' => false,
    ),
  ),

Operating system:
Arch Linux

Web server:
nginx

Database:
mariaDb

PHP version:
7.2.0

Nextcloud version: (see Nextcloud admin page)
13.0.0 Beta 3

Updated from an older Nextcloud/ownCloud or fresh install:
Updated from 13.0.0 Beta 1, Version 12 was installed before.

Are you using external storage, if yes which one: none

Are you using encryption: no

Are you using an external user-backend, if yes which one: none