Issue when combining Apache2, PHP 7.4 FPM and DomPDF

Theory!

Possibly output buffering is enabled in php.
For this reason, the header set by the library is replaced by the header from the Response class.

Look in the php config for the output_buffering option. And if it's on, turn it off.

Or you can try another solution. Remove header!

PHP Code:

//in controller method
$this->response->removeHeader('Content-Type');