With an Arduino Uno R4 WiFi.
I uploaded a sketch. Inside a loop, there was an instruction to write to the serial monitor, but I had incorrectly commented out a wait line. When it was executed, something obviously happened on the serial monitor or on the board. When I restarted the board, the problem started. The L LED (D13) keeps blinking softly, and a moment later, the RX LED blinks, and then the board disconnects and reconnects, and it returns to the same state. This happens every 3-4 seconds. So, to upload another sketch, the compilation time must be less than that to be able to upload it to the board. If there's enough time, the board goes into upload mode and receives the sketch.
Now, if I upload another sketch, for example, something to manipulate the LED matrix, it uploads, and after restarting the board, it works fine. But if the board was in the state I explained before, it must be a quick build to give it time to upload, and sometimes it remains in that state until the board is rebooted.
I removed everything related to the serial terminal from the initial code and re-uploaded it (the way I explained), and if I close the IDE and reboot the board, it works fine. But as soon as I reopen that project in the IDE, with the board connected, the board goes into the unstable state.
I took the LED matrix project, which works fine. I added an instruction to write a line at the beginning to the serial monitor, and the same thing started happening. Luckily, after removing that change and closing the terminal, it didn't happen anymore.
I tested with a dirfferent terminal software, Teraterm, and the problem don't happen.
Why is this happening? Those extremely fast writes to the terminal at the start, have they messed up something somewhere?