Hi.
With PHP7.4 you need to change the Apache settings.
https://httpd.apache.org/docs/2.4/ja/mod/core.html#limitrequestline
The following is an example of Nginx, but you can update the Apache configuration file in the same way during the initialization process of App Service.
https://dev.classmethod.jp/articles/app-service-linux-php-default-document/