Verify platform produces warning with "composer executable was not found"

After upgrading aegir 3.11 to 3.144, all platform verifies produce the following warning

The composer executable was not found. Install composer using instructions located at https://getcomposer.org/doc/00-intro.md#globally or, if you wish to disable composer install, set "provision_composer_install_platforms" to FALSE in the ~/.drush/drushrc.php file.

Composer Version 1.6.2 is installed globally at /usr/local/bin/composer, 755 permissions.
Drush Version 8.1.16 is installed globally at /usr/local/bin/drush

Running composer manually works fine, from inside a platform or outside. None of the platforms have plat-specific composer or drush installed.

The message is produced when verifying any platform (even the hostmaster platform) and the platforms are still usable, just the tasks turn yellow.

Putting

$options['provision_composer_install_platforms'] = FALSE;

into ~/.drush/local.drushrc.php suppresses the warning.

This was on an Ubuntu 14.04.5 server with PHP 5.5, but I also got the warning on a Debian server (slightly newer PHP 5.6)
Actually, I don't recall seeing this warning on our newest server with PHP 7, which was already upgraded to Aegir 3.144 when I started this upgrade-all-the-things adventure.