I repair these all the time. Usually mode of failure is one LED failing. They are connected in series, so the lamp goes dark or flickers. The black dot on the yellow substrate dot is typical for the faulty ones.
Just use a lab power supply set to 30mA constant current with some probes to test them individually.
Put the PCB on a hot plate (using a soldering iron on these aluminum PCB does not work well) and replace the bad chip.
Measure the current and voltage on a known good sample to identify the LED. You can find the LEDs on Ali, Evilbay etc. Example:
https://www.ebay.de/itm/224072197741
Thank you, sir, for the effort of a really useful point. I love your user name. It's a good reminder that "Ohm's Law" is only a small signal approximation in most situations and was called a "LAW" by what the late prophet George Carlin used to call Symbol-Minded.
First, I guess a few mAs In-circuit application is valid in this case, I'll report to you when I do that, even though I can tell from voltage measurement in all 17 LEDs, in addition to the tiny black dot that you mentioned too which one (and only one, as you referred to and as I can see in the case in front of me) is bad.
How hot is a hot-plate supposed to be here? Not looking for a number with fractions, but is it as hot as deep frying on the hot plate (real hot, red) or barely ON?
And before doing that, how do you even take the aluminum/PCB out on the first place? (there are 4 supply pins that are holding it to the base that are not soldered but attached using black magic)
I think it's not worth all that effort anyway, and just shunting the bad LED-rectangle that is bad with something is adequate. Thanks to you now I know that they call them "SMD LED 2835 Chips" and it's good to have 100 for future repairs(5 cents a piece) but still the choice of voltage and color needs some thinking.