How to fix unstable PS?
The situation is more clear now.

I expect that the incomming mains supply causes the problem and that switching the bulb creates a voltage drop high enough to make the power supply restart. 

Remember that the led bulb has a very high inrush current due to the rectifier on the input of the build in converter. This could be more than 20 A. The connecting cable could be the cause 

To verify this.


Try a short and big connection cable.
Separate the two systems and feed the buck converter from a different outlet.
Try a standard bulb.


PS. I use the same power supplies and have no problems.