I had that going on for a while, then it went away but now it's back.
I cut my products down to things without protein, quats, even water soluble cones went but I still have it.
I started rinsing upside down and I've been diffusing are the only things I can think of that I'm still doing. I cut down my diffusing time and I make sure I don't diffuse that spot.

Now another weird thing I noticed. Product seems to get trapped there. So I assume my cuticles are open more?

I wish I knew what to tell you but since I switched to different products my hair is way curlier but that tangly spot at my nape on the underneath is still there. I think I might have to give up rinsing upside down to see if that's the culprit.

Sorry I wasn't more help hon
Low porosity, medium texture.

If it smells good put it in your hair, if it tastes good spit it out!