Not sure if this helps you at all, as I'm a bit late to the party, but I stumbled on this post looking for solutions to a very similar issue. I'll put my experience here to help you and others that find this post.
I upgraded my setup from WS2812B to WS2815 about a month ago, and everything seemed to be working fine. They quickly developed intermittent flashing issues, and that grew into complete unresponsiveness, random flashing, and random colors displayed when setting it via the app color palette. I figured this must be a bad hardware issue, and I tried replacing just about every component in my setup without any luck.
Long story short, the issue was caused by my cable management, as grouping the TV power, 12V supply, and HDMI cables together with the WS2815 data line caused EM inductive interference. Separating the wires running to the LED strips and the wires feeding power to everything else immediately resolved all my flashing issues. Hopefully that helps, as your symptoms sound eerily similar to what I was dealing with.
PS. I found a rate of 3800000 to be the sweet spot but YMMV.