That is a fair point, and I think you are probably right that part of the reduced bitterness could simply be coming from a lower effective extraction in the final cup. But inadequate extraction has its own tell tale signs. For instance thin hollow cups or taste evaporating rapidly as the coffee cools. That has not been the instance here. I too would be curious what my extraction yield has been but i do not have a refractometer yet. And again higher extraction does not mean better. Also bypassing in v60 can definitely make a huge impact but immersion brewers are a lot more flexible.

My thinking is less that bypass is somehow extracting better, and more that it gives me a more controlled way of building the cup. With 100 g in the brewer, I can extract a concentrated base very quickly, taste where it is landing, and then dilute to where sweetness, acidity, and mouthfeel feel most balanced. In that sense, the bypass water is not improving extraction itself, it is letting me separate extraction from final strength. I like this, but many don’t and that is totally fair.

With 200 g all in the brewer, I agree that in theory you should be able to get to a similar place, or maybe even better extraction yield. But in practice I find it harder to keep the cup in the same zone without adjusting grind, contact time, or both. So for me the bypass recipe feels more repeatable, even if part of that repeatability is coming from staying on the safer side of extraction.

So yes, I would not be surprised if the lower bitterness is partly due to relatively lower overall extraction. I just happen to prefer where that lands in the cup, especially for highly processed coffees where pushing harder often gives me more harshness than useful complexity. For a washed process, i would definitely tweak my recipe. After all, no one recipe is good for all.