Why does Fpdi fail to extend TCPDF when using PHP8.1? - tcpdf

Question

I'm using:
TCPDF version 6.4.4.
FPDI version 2.3.6
PHP Version 8.1.7 (Running on Apache and Windows Server 2012,a dev environment)
And, I don't think it's relevant but I'm also using
tcpdf-extension
These were installed with composer:
{
"require": {
"tecnickcom/tcpdf": "6.4.4",
"setasign/fpdi": "^2.0",
"naitsirch/tcpdf-extension": "dev-master"
}
}
I am also using a Java/PHP bridge. This is relevant because it changes the error message, however, I doubt this is the cause of the issue.
The problem I'm having is that Fpdi fails to extend TCPDF.
The error message I get looks like this:
PHP Fatal error: During class fetch: Uncaught Error: Call to a member function invokeBegin()
on null in http://127.0.0.1:8080/JavaBridge/java/Java.inc:557
Stack trace:
#0 http://127.0.0.1:8080/JavaBridge/java/Java.inc(56): java_Client->invokeMethod(0, 'typeExists', Array)
#1 C:\Program Files (x86)\PHP\v8.1\vendor\setasign\fpdi\src\Tcpdf\Fpdi.php(33):
java_autoload_function('setasign\\Fpdi\\F...')
...
#8 {main} in C:\Program Files (x86)\PHP\v8.1\vendor\setasign\fpdi\src\Tcpdf\Fpdi.php on line 33
It is failing to fetch a class and the php/java bridge, as a last ditch effort to load it, is attempting to fetch the missing class with a java autoloader, which it fails to do because it's not a java class. Unfortunately, it doesn't give me the full name of the class that's it is trying to load. But this error occurs in setasign\fpdi\src\Tcpdf\Fpdi.php on line 33. Which reads:
class Fpdi extends \TCPDF
My code that invokes this looks like this:
define("COMPOSER_AUTOLOADER","C:\\Program Files (x86)\\PHP\\v8.1\\vendor\\autoload.php");
require_once(COMPOSER_AUTOLOADER);
use setasign\Fpdi\Tcpdf\Fpdi;
class PDF extends Fpdi {
...//functions related to my use case
}
My code that calls this extended class:
// initiate PDF
$pdf = new PDF($data['orientation'], 'mm', USLETTER, true, 'UTF-8', false);
$pdf->setPrintHeader(false);
$pdf->setPrintFooter(false);
$pdf->SetDefaultMonospacedFont(PDF_FONT_MONOSPACED);
$pdf->SetDisplayMode('real');
$pdf->SetAutoPageBreak(true, 1);
//set margins
$pdf->SetMargins(0,0);
$pdf->SetHeaderMargin(0);
$pdf->SetFooterMargin(0);
...
$pdf->AddPage($data['orientation']);
...
$pdf->Output('newpdf.pdf', 'I');
Also probably relevant, I set up a test that uses only the TCPDF library without Fpdi and it fails to load the constants that are supposed to be set up in TCPDF_static.php. (USLETTER for instance), but if I manually define that constant, it does load the PDF as expected:
require_once("C:\\Program Files (x86)\\PHP\\v8.1\\vendor\\autoload.php");
use setasign\Fpdi\Tcpdf\Fpdi;
define ('USLETTER',array( 612.000, 792.000));
$pdf = new TCPDF('', 'in', USLETTER, true, 'UTF-8', false);
...
$pdf->AddPage();
...
$pdf->Output('newpdf.pdf', 'I');
Another detail: this code was working fine with PHP 5.6 and some older versions of TCPDF (6.2.11) and Fpdi (1.4.2). Upgrading to PHP 8.1.7 required updating these libraries.
Why is TCPDF failing to load its constants and, more importantly, why can't Fpdi extend TCPDF?