[Bug]: Some CSS refuses to load, errors in browser console, when using custom_apps folder

[fulldecent]

⚠️ This issue respects the following points: ⚠️

• [X] This is a bug, not a question or a configuration/webserver/proxy issue.
• [X] This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• [X] Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• [X] I agree to follow Nextcloud's Code of Conduct.

Bug description

Missing CSS resources

Steps to reproduce

Install these apps

Access the tasks app

Expected behavior

No console errors.

Got this error:

Refused to apply style from ... custom_apps ... because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.

However this is actually just an error that that file does not exist.

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Related: https://help.nextcloud.com/t/troube-with-custom-app-directory/139536/3

Installation method

Official All-in-One appliance

Nextcloud Server version

28

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• [ ] Default user-backend (database)
• [ ] LDAP/ Active Directory
• [ ] SSO - SAML
• [ ] Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response