Hello,
I'm going to put my neck on the block here and suggest that the 1000uF on the output of your buck converter is certainly a big capacitor and would probably power the neopixel strips (especially as you're only sing quite short ones) for a second of so after the power is turned off.
What's inside your buck converter? Could you get away with using something simpler like a Voltage regulator? A 7805 would do the job - I'm using one to power a speedo signal generator in a van, so it's happily bringing approx. 14V down to 5V - you might not need such a big smoothing capacitor (or any capacitor), then.
What's the job(s) of the 220uF's across the neopixel strips? I've built a few projects with noepixel strips (I built a light-up tube for a pedal-powered generator to show how much electricity people were generating at a science fair - that had 150 pixels in it) and have never put capacitors across the supply.
I've had neopixels light up the wrong colour or light up unexpectedly, but it's always been a bad earth or dodgy code for me...