Mikrotik still writes (something?) to serial port even when console is disabled

RouterOS General
1

We’re experiencing one particular problem when using RB433 for remote serial port access to Extreme switch.

When both Mikrotik and Extreme power cycle simultaneously, Mikrotik will cause Extreme switch to enter boot loader prompt by sending output to the serial port.

Even if console output is disabled with /system routerboard settings set baud-rate=off as described here (https://wiki.mikrotik.com/wiki/Silent_boot#RouterOS_configuration), Extreme still fails to boot beyond loader.

Inspecting console port output from PC with `cat /dev/serialportname’ does not reveal any output.

Is this Mikrotik’s behaviour a bug?
How to disable output to serial port completely?

2

It prints out some information while booting. I’m afraid it cannot be prevented.

3

But there’s no visible output during the boot process when baud-rate=off.
What does it print then?

Anyway, current behavior contradicts what docs say(i.e., the boot isn’t “Silent”).
I think it should be corrected.