Rewiring USB LED lights to directly use a 5V DC-DC Converter
It is difficult to say what is happening with out measuring the voltage before and after the modifications.

Consider what may happen if the load on the replacement power supply is too low.  A 12 to 5 volt DC to DC converter is likely a buck switching power supply.  Such supplies may require a minimum load to operate correctly.  This is discussed in detail in this stackexchance question.