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Probably a broken trace/resistor/soldering to FSET, ISET or FILTER pins. Measure on diode mode the resistors connecting to them to see if they have continuity to the LP8550.
Resistors and traces were intact! Incorrectly soldered LP8550 chip. Backlight now works normally.
I suspect I pushed on the chip while the solder was still liquid, under the microscope the chip was slightly tilted and the balls were flattened.