Mine looks just like your 48 SMD Flat Panel LEDs , and I do have some like that in a 36 panel that work fine in all the fixtures. The ones that don't work are about 1 1/4" long and 1/2" wide about as thick as a finger. They have 13 led lights on them and don't trail the long plug in cord as the flat panel ones do. I have 6 pair, and only 1 pair work in one of the light fixtures. The other 2 pair I want to put into a single light fixture over the chairs in the rear of the trailer and over the bed in the upper 5th wheel bedroom. The 36 flat panel lights work fine in both fixtures, but the round ones don't picture on ebay similar to this one:
I have extra flat panel led lights, but the adhesive that holds them to the fixture doesn't adhere to the fixture and after being on for a few minutes drop onto the clear lens. I have tried industrial strength Velcro that sticks up with the same problems with the adhesive failing. The finger length leds would fit in without the Velcro and the trailing wire.
Looks like I will have to dig out the voltmeter.