Oh ok, now i am getting it finally. You do have a smps that is meant to output 12v, and can be used for anything, incl leds that have resistors in series with a value suitable for 12 supply. Sry i didnt got that from the first time, its coz this is very terrible efficiency wise, e.g. 3-3.5v led drop means ~75% of output is wasted on series resistors, not to mention smps losses, so probably just 10-20% is actual power on leds. This is why when you said "led supply smps" i just skipped and imagined a setup of a "led driver" ics that does just that without series resistors - which is something else. Sry for the previous then and my bad.
Then you have a regular smps with a single 12v output. It does make 12v with no load, and then voltage falls down under load.
Perhaps the ic that regulates the duty cycle fails to increase it as response to load changing. If the duty doesnt increase then ofc the voltage falls like that. Sry i have never dealt with that kind of smps, so cant say much, just speculation. But that could be a valid culprit. The ic that gets the feedback from output, and then it outputs the duty cycle for the switch. This one could use checking, e.g. check its duty output when there is no load, then put a load and see if its duty changes, e.g. if it reacts to load changing then it works, otherwise it doesnt. Also if the load changing does produce a change in feedback level for that ic to see it in order to react, e.g. the ic might be ok but it doesnt register a feedback path change. Could see its duty cycle with a scope. Perhaps with a true rms dmm, those readings even might be incorrect but at least should show a change if there is one. Or if you get some supplies like that which do work, try replace that ic with one that is confirmed to work, and perhaps do that with other parts. Btw, if you do anything while its powered up, do put a light bulb in series with the mains voltage, that would limit the power into the smps and might help in case of unfortunate shorts.
Then you have a regular smps with a single 12v output. It does make 12v with no load, and then voltage falls down under load.
Perhaps the ic that regulates the duty cycle fails to increase it as response to load changing. If the duty doesnt increase then ofc the voltage falls like that. Sry i have never dealt with that kind of smps, so cant say much, just speculation. But that could be a valid culprit. The ic that gets the feedback from output, and then it outputs the duty cycle for the switch. This one could use checking, e.g. check its duty output when there is no load, then put a load and see if its duty changes, e.g. if it reacts to load changing then it works, otherwise it doesnt. Also if the load changing does produce a change in feedback level for that ic to see it in order to react, e.g. the ic might be ok but it doesnt register a feedback path change. Could see its duty cycle with a scope. Perhaps with a true rms dmm, those readings even might be incorrect but at least should show a change if there is one. Or if you get some supplies like that which do work, try replace that ic with one that is confirmed to work, and perhaps do that with other parts. Btw, if you do anything while its powered up, do put a light bulb in series with the mains voltage, that would limit the power into the smps and might help in case of unfortunate shorts.