You have created 2 tasks with priority 0 which is the lowest priority in the system. ESP IDF implicitly creates a several other tasks (many with higher priorities) and your tasks have to share CPU cores with those.

Firstly there are 2 idle tasks (one per core) with priority 0. Since the priorities of your tasks and the idle tasks are equal, they share core time equally (50/50). So the scheduler rotates them on each core every 10 ms by default, which is quite likely the source of your flickering.

Then there are other, higher priority tasks which ESP IDF creates. If you enabled WiFi then its driver runs in a high priority (23 by default) task. The High Resolution Timer service creates its own high priority (22) task. If you've enabled TCP/IP networking, this has runs in a high priority task; etc. Granted, those tasks don't take up much core time if you don't use their services. But they take up some.

So, in short you don't get exclusive use of a CPU core. You can cut out the idle tasks by raising your priority (by the way, the Task Watchdog Timer will complain and reset the system soon if you don't let the idle task run at all; unless you disable it). You can also cut out all other tasks by raising your priority to the maximum, but in any real embedded system those other tasks do something useful ;)