My stock is too sweet As part of my Thanksgiving preparation, I made a chicken and turkey stock by first roasting the bones and then cooking them very low for a few hours. About an hour or so before the end of their cooking, I added roasted vegetables -- a combination of onion, carrot, celery, parsnip, turnip, and parsley root. I didn't add any other flavoring other than a touch of salt. (I know most people don't salt their stocks, but I tend to undersalt everything, so adding it at every step will help.)

My stock is tasty, but rather sweet. Is this a factor of too many sweet vegetables (or not enough celery)? Or is there something else I'm missing?

(I compensated in my gravy by adding a touch of soy sauce and fish sauce to add more umami flavoring. But I'd like to understand why it happened.)
Some vegetables are aromatics for infusing flavor without changing the underlying base, and certain combinations become known for cuisines.  Onion, celery and carrots make mire-poix.  Onions, garlic and tomato make sofrito.

Turnips and parsnips are not those kinds of vegetables, usually.  Parsnips get very sweet (a good thing usually) as do onions and carrots.  You just combined a lot of the ones that get sweet and intensified it by roasting.

If you are looking for a more neutral broth, stay with onions, celery and carrots and simmer, don't roast.