Hi Claire, I have updated report from the battle field 
I was not able to shut down driver directly from PIC output pin even in OD mode. So I decided to modify my schematic as bellow:
One N-MOSFET, 10k resistor on gate (just in case), 100k pull up resistor to +12V and voila !
It’s working like a charm.
The soldering was tricky as I had on my drawer only smd mosfet, resistors are 805, but the result on pcb is almost invisible 
In the future projects I will be not even trying to tie SHDN directly to PIC outputs. Lesson learned! But I still have no idea why it doesn’t work?
From my perspective we can close the topic. Thank you for your effort.
Sławek