None of the global page messages are appearing on any page in the admin panel. This includes and is not limited to my own custom messages, saving a product, canceling an order and clearing the cache.
I've tried with all caches enabled and disabled with no luck.
Some investigation has shown that I can get messages showing by adding
$clear = false; in the
getMessages() function in
vendor/magento/framework/Message/Manager.php on line 120. This tells me that messages are being cleared on every redirect without allowing the latest added message to display.
I can't tell what happened to the system before I started seeing this bug.