Suppress libreoffice headless command outputs which are not via stdout/stderr

Question

Running the following command is outputting to unix terminal, even if stdout/stderr are redirected to a file.

Both commands:

libreoffice --headless --convert-to xlsx <my_file> > 1.tmp
libreoffice --headless --convert-to xlsx <my_file> 2> 1.tmp

Are printing the following to the terminal (and not to 1.tmp):

convert <my_file> using filter : Calc Office Open XML
Overwriting: <out_file>

1. How can it be that a process prints to the terminal not via stdin/stderr?
2. How can I suppress those printing? (to be more precise, I'm trying to suppress them through python's subprocess, using stdin=DEVNULL and stdout=DEVNULL - which is unsuccessful).