It seems like there is a drafting effect that slows a passing rider down when going uphill. Today I was riding at 3.6 w/kg behind a rider at 3.1 w/kg up the climb and was not catching them (I was at the tail end of the drafting window).
I also noticed distinct slowdowns on the climb when passing other riders at constant power. I’m guessing this is a drafting tweak to make it easier to stay behind other riders, but maybe it should be lessened or removed when going at slow speeds.
Just to mention that the effect has been reintroduced for quite a while now and is clearly adversely affecting group dynamics. On the other hand TT bikes are much less prone to that issue (and are drafting as well) making high intensity group TT rides much more enjoyable.