I have my strip split into four segments for my different cabinets in the kitchen. The first two are on one side of the room and then there is a ~5m wire run up through the ceiling and back down where it hooks up to the other two segments. I am powering both ends of the strip with 12V from a 12V 5A power supply, but the pixels after the long run (not immediately after, but within 4 or 5 pixels) are not controllable via my software. They also do not turn off once they are on. I have noticed as well (not included in video) is that if the brightness of the pixels is kept low that the entire strip works, but once it is brought above some threshold all control is lost.
In total there is 56 drivers driving 168 pixels. The first 20 or so work fine, then they stop. Broken into segments of 8/8/11/29. There is approximately 10m of 20awg wire between the segments, with the largest run of wire being 5m as stated before.
Since you mentioned it all works if the brightness is kept low, before taking the time to put a null pixel in the mix, first check your voltages (while running a higher brightness level when you see the problem) before and after that ~5m wire. If the voltage has dropped too much maybe that’s the issue and you need to run an additional 12V supply to the middle there somewhere.
I ran another power line to the start of the segment where the issue begins (in addition to powering the start and end of the strips) and it fixed the issue for the most part. The voltage drop is still a small issue, but due to the load in the entire circuit and the distance my power lines have to run to get to that side of the kitchen, I can’t get more than ~10.5V to the far segments. I’ve ordered a second power supply to place on the offending side of the circuit so I can get true 12V to these strips and I think that should do the trick.
DC-DC stepdowns are very inexpensive. Consider going to a cheap 24 DC supply, and then having 24->12 step-downs where you need them. That is, run a 24V power rail along your LEDs, and tap with 12V step-downs periodically to power the strip.