The question is:
If I communicate with any UCCI engine I have (other than UFX) everything is OK

But in case of UFX engine, it tends to play on the same side as I.

But, if I connect UFX engine to WinBoard, everything is working just fine.
I have examined UCCI2WB code in order to determine what been sent to the stdin of engine, but did not find anything special.
Does anyone have a clue what I am missing?
Engines are here in case anyone needs them
https://easyupload.io/sybp7k