Email formatting stopped working after Ubuntu upgrade with CakePHP 2.10.13 / PHP 7.3.3

As part of recent maintenance, a legacy box was upgraded from Ubuntu 16 to 18.04.6

This has caused the HTML email notifications sent from the box to lose their inline formatting. None of the templates or config has changed. The emails still send OK.

I can not find any reported issues on this upgrade related to CakePHP 2.10.13 or PHP 7.3.3.

I’d appreciate any help in understanding how the templates are complied for sending in the version or known issues with this combination stack.