It is intended to work for 12V/300 mA.I am not confident that the circuit in post #1 works as intended. I am guessing that it is intended to be a 12.0 volt power supply able to deliver up to 300mA.
Given that TI and others have published reference designs available on line, along with detailed explanations, it makes sense to see what is offered there for free.
I don’t see anything wrong with that feedback circuit. it should regulate to 12.07V with those values, so there won’t be any LED current at 12V because there is only 2.48V on the adj pin.hi SG,
This is what LTspice shows for TL431 circuit, a problem.
E
Ref PDF Page #24