Custom fonts php-font-lib AdobeFontMetrics issue - wants to save files, no write access

So... after moving to a strictly-controlled host (there's no write access anywhere except /tmp and a local /log), I get this fatal error.

The fonts look all wonky, too. Looks like it's being rendered from UTF8 by a non-UTF-aware program, every letter is rendered k i n d a l i k e t h i s.

It looks like it's trying to write to some file (as evidenced by fwrite), but there will be no write permissions for PHP into the code directory, that's out of the question.

Ideas?

Stack trace:

PHP Fatal error:  Uncaught TypeError: fwrite(): Argument #1 ($stream) must be of type resource, bool given in /var/www/api/vendor/phenx/php-font-lib/src/FontLib/AdobeFontMetrics.php:191
Stack trace:
#0 /var/www/api/vendor/phenx/php-font-lib/src/FontLib/AdobeFontMetrics.php(191): fwrite()
#1 /var/www/api/vendor/phenx/php-font-lib/src/FontLib/AdobeFontMetrics.php(211): FontLib\\AdobeFontMetrics->addLine()
#2 /var/www/api/vendor/phenx/php-font-lib/src/FontLib/AdobeFontMetrics.php(49): FontLib\\AdobeFontMetrics->startSection()
#3 /var/www/api/vendor/phenx/php-font-lib/src/FontLib/TrueType/File.php(361): FontLib\\AdobeFontMetrics->write()
#4 /var/www/api/vendor/dompdf/dompdf/src/FontMetrics.php(245): FontLib\\TrueType\\File->saveAdobeFontMetrics()
#5 /var/www/api/vendor/dompdf/dompdf/src/Css/Stylesheet.php(1511): Dompdf\\FontMetrics->registerFont()
#6 /var/www/api/vendor/dompdf/dompdf/src/Css/Stylesheet.php(1368): Dompdf\\Css\\Stylesheet->_parse_font_face()
#7 /var/www/api/vendor/dompdf/dompdf/src/Css/Stylesheet.php(302): Dompdf\\Css\\Stylesheet->_parse_css()
#8 /var/www/api/vendor/dompdf/dompdf/src/Dompdf.php(617): Dompdf\\Css\\Stylesheet->load_css()
#9 /var/www/api/vendor/dompdf/dompdf/src/Dompdf.php(695): Dompdf\\Dompdf->processHtml()
#10 /var/www/api/api-modules/func.pdf.php(294): Dompdf\\Dompdf->render()
#11 /var/www/api/api-modules/v1/rejestracja.php(131): generateAndReturnWarrantiesPDF()
#12 /var/www/api/api.php(245): include_once('...')
#13 /var/www/public/api.php(11): require('...')
#14 {main}
  thrown in /var/www/api/vendor/phenx/php-font-lib/src/FontLib/AdobeFontMetrics.php on line 191

Start with vendor/phenx/php-font-lib/src/FontLib/AdobeFontMetrics.php at line 191 and trace the call from dompdf/src/FontMetrics.php:245 through CSS font registration. Reproduce the failure on a host where only /tmp is writable, then determine the expected writable location and font-encoding behavior. Done requires a confirmed handling path and regression coverage, but the issue does not specify the intended fix.