c++ should condition variable be notified under lock

You do not need to notify under lock. However, since notify is logically happening when the actual value is changed (otherwise, why would you notify?) and that change must happen under lock, it is often done within the lock.

There would be no practical observable difference.

if I understand your question correctly, it's equivilant to "should the notifier thread lock the mutex while trying to notify some CV in other thread"

no, it is not mandatory and even does some counter-effect.
when condition_variable is notified from another thread it tries to-relock the mutex on which it was put to sleep. locking that mutex from its working thread will block the other thread which trying to lock it, untill the that lock-wrapper gets out of scope.

PS
if you do remove the locking from the function that send data to the worker threads, ready and processed should at least be atomics. currently they are synchronized by the lock , but when you remove the lock they cease to be thread-safe

If you do not wait on a condition variable then the notification is lost. It does not matter if you are holding any locks. A condition variable is a synchronization primitive and do not need a lock for protection.

You can miss signals with and without lock. The mutex protects only normal data like ready or processed.