Very interesting.
I’ve just completed an upgrade and got this error on the Stackable plugin instead.
I also upgraded TablePress to 2.2.2 in the same batch after updating Stackable.
This makes me wonder if this is not specific to a plugin but has something to do with WordPress and/or WPEngine (we are also with WPEngine).
Uncaught Error: Call to undefined function fs_get_optional_constant() in /nas/content/live/site/wp-content/plugins/stackable-ultimate-gutenberg-blocks/freemius/includes/class-freemius.php:1361
Stack trace:
#0 /nas/content/live/site/wp-includes/class-wp-hook.php(310): Freemius->_run_garbage_collector('')
#1 /nas/content/live/site/wp-includes/class-wp-hook.php(334): WP_Hook->apply_filters(NULL, Array)
#2 /nas/content/live/site/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#3 /nas/content/live/site/wp-settings.php(495): do_action('plugins_loaded')
#4 /nas/content/live/site/wp-config.php(120): require_once('/nas/content/li...')
#5 /nas/content/live/site/wp-load.php(50): require_once('/nas/content/li...')
#6 /nas/content/live/site/wp-admin/admin-ajax.php(22): require_once('/nas/content/li...')
#7 {main}
thrown
