Won't boot without UART and monitor app

[marware]

Hi,

We are using ESP32-WROOM-32UE board on custom PCB.
After the first batch of 20 devices, we encounter an issue on two boards.
Both boards have always reproducible issues, but one worked few days and then manifest the problem while other wont work from the beginning.
So, the problem is that esp won't boot without connecting the serial cable and starting 'idf.py monitor' app.
As soon as the monitor starts, the device boots normally, keeps sending the logs, and works without issues.
It works also after flashing with cms: "idf.py -p COMX flash monitor".

It seems that something is preventing esp from booting.

Do you have any idea what could be causing the problem and in what direction to investigate further.
Thanks in advance.

[ESP_Sprite]

Check the reset and IO0 pins. On poweron, the reset should go high after the 3.3V stabilizes (see ESP32 datasheet) and IO0 should be high to not go into download mode. Easy check if this is the issue is to manually force IO0 high and reset the ESP32, see if it comes up then.