Data issues (noise?)

I have 3x NodeMCUs in a box driving LED strings (ws2811, 12v, <250 nodes each w/ power injection on ends) on my deck. One runs the strings on the top rail, one on the bottom rail and one the vertical columns every 3-4’. I’m running into a strange issue of occasional flickering on all three only when all three are running a sequence. No problems occur when I run each individually; same when running only two, no issues. Similarly, no issues when I just set all to solid color. I’m taking the data directly from each NodeMCU currently (~4ft to first LED); strangely the issue is further amplified when I boost the data to 5v (i.e even more flickering). I’m really struggling to find the way around this, so any thoughts as to what could be going on is greatly appreciated.

What are you using to “boost” data?

Grounds all connected to each other?

All grounds are common, yes.

When boosting the data signals, I was tied into a simple logic level converter:

I believe those are not fast/good enough for wled use. Check the wiki for suggested parts.

Did you ever get to the bottom of this. I have exactly the same issue. 3 in the same and get lots of flickering. I have resorted to having 3 small boxes remotely, and using the larger box as a psu distribution for the 12v PSU.
This solution works fine, but would be interested to find out if there was a way to fix the issue for the future.