Profile picture uploads fail when using a preparse field in an user

[mikesnoeren]

I am working on a website where all users have a public profile that is editeable by the user from the front-end.
Because users don't have slugs natively I decided to generate one using a preparse field.

The field contains the following twig code:

{{ (element.fullName ?? element.id)|kebab|ascii }}

And in my code I use the following, which checks if current user belongs to current page.

{% set user_on_this_page = craft.users.userSlug(userSlug ?? currentUser.userSlug).one() %}
{% if not user_on_this_page %}
    {% exit 404 %}
{% endif %}

The code above works fine, but the edit-profile form doesn't.
As long as the preparse field is present I am unable to upload a picture to craft, so users can't change their profile pictures, all other form functionalities work fine..

I've opened a support ticket with Craft and they confirmed the issue is within the preparse field and then asked me to make this bug report.

If needed for debugging, I am willing to send my project file, edit-profile template and composer files.