Loading Kirki via composer messes with other plugins/classes

[JiveDig]

Issue description:

When Kirki is loaded via composer (autoload.php) it causes clashes with other plugins, in a very strange way.

Version used:

4.1.0

Using theme_mods or options?

Neither, it happens when you load Kirki itself.

PHP error messages that might be related

Class 'Tutor\Models\CourseModel' not found

Via: https://wordpress.org/plugins/tutor/ on the Dashboard.

Class 'Smartcrawl_Vendor\Vanderlee\Syllable\Syllable' not found

via: https://wordpress.org/plugins/smartcrawl-seo/ when editing any post/page.

It happens with various other plugins as well, and doesn't happen when Kirki is not included.

Code to reproduce the issue (config + field(s))

I created a blank plugin, and installed Kirki via packagist via:

composer require kirki-framework/kirki

Then, the only line of code in the plugin is:

require_once __DIR__ . '/vendor/autoload.php';

When some plugins like Smartcrawl SEO, Tutor LMS, and others are loaded, it throws class not found errors, even though the class/file is definitely there. Commenting out the require_once line makes the error go away.

Another dev/friend said it has to do with how Kirki is using include-path, but I have yet to pinpoint the exact source of the issue.