God I hate my stupid brain sometimes.
So yes... that is exactly what it was....
In my c++ I run these smoothers for some parameters. To avoid discontinuities when turning a parameter. You generally don't want to flush 1000's of parameter updates on things like delay times, so you smooth it out over time. Maybe only 10ms, but it helps to make it sound better and brings down CPU too. Cool.
And that was all well and good. Worked fine.
But in my setParameter callbacks, I was only setting the target. I wasn't setting the initial value. Which now I read it, is literally the dumbest stupidest mistake that I could've made.
Time for more Guinness.