Led bulb lights up even when 1 wire is connected. How?
This is power line electric field picked up open circuit wire. Proximity effects and stray capacitance close the loop to permit trace visible light on the order of xxuA is possible and is certainly visible.  Normally common mode signals follow the LED  wiring from any possible effect by electromagnetic coupling on 1 side and conductive coupling on the other.

The open circuit lightstrip acts as an antenna to AC electric field and rectifies via stray capacitance coupled E field also coupled as CM high frequency noise across ferrite transformers in SMPS.

The exact paths of signal coupling would require a full description of your components, layout and proximity of E fields etc.