Thanks a lot for your quick reply. It was very helpfully!
I did some more testing and in the end got another shelly 2.5 to compare. Unfortunatelly I have no test bench to test it properly with DC input, but:
With AC 230V input and no wires connected to any SW or O ports, I also get 130V between N and SW1 / SW2. Issue starts when I connect my wires to SW ports. It seems that 130V signal sent to SW is so weak that simple wire resistance is causing issues for it, as my SW wires are 7m long and having 3 wago connectors on each SW wire (7m distance from switch to shelly ). When I mount shelly just next to switch (SW wires are 10-15cm long), it behaves fine. As soon as SW wire is longer and having wago connector on it, no way, it just switches N-SW voltages to 160V/230V. I am guessing that SW signal sent by shelly is so weak it is disrupted with smallest wire / connector resistance.
When trying with new shelly I got - works fine from start with full 7m legth of SW wires. However voltages with SW wires connected are 80V and 110V AC.
Is there any specification telling how long SW wire can be and should I send my Shelly back to shop as defective?
Thanks.