@Vitaliy
My guess is these are the new bus-addressable LEDs. I should have asked! But I’ve seen the same on my bus-addressable LEDs in certain conditions. They are ws2812 compatible but have 3 pins, no data output. They have an address programmed internally. Your can find more in this forum post.
Yes if you terminate a transmission line (to gnd?), but that would also cut the voltage in half if around 100 ohms (PB driver has 100 ohm) right?
1k on the PB output will increase slew rate which can reduce reflections magnitude. Also 1k in known to work for 2812, though thinking about it the fan out for this many might need less.