Possible soap/dandruff buildup?
You could have something there with the tightening spray idea! It seems like it could be a mixture of that and residue from what I'm now seeing.
I found another brand of shampoo from a small shop that claims to gently remove buildup over time, strengthen dreads and give them back the oils they need (all vegan/small batch made, their freakin amazing) and after a deep clean and 3 washes with this new shampoo, the dreads seem to be slowly softening up and becoming less dense. Not 100% sure if it's because they were too dry from tightening sprays, or if the slightly clammy/sticky feeling at the end of my dreads after rinsing justifies the residue idea.
If I squeeze really hard (because this dread literally has almost no give from how tight it is, at least right now) I can feel a slight stickiness in the dread, but no smell comes through. I think my current assumptions is the locking spray tightened them a little tighter than I'd hoped, and it made it easier for buildup to occur slowly over time. this is my first set of dreadlocks so I greatly appreciate you sharing your experiences!