Our family had a very similar experience. I finally gave up on my granddaughter's 2019 return. Her mother took over the task and, like you, managed to get a disappointingly small reduction in the girl's taxes but a few months later got a huge refund from the IRS with no explanation.
The painful experience was 2018, but that was a result of the 2017 tax changes and no fault of TurboTax's. But I agree: TTax did a miserable job with the 2019 kiddie tax when using the parents' tax rates. 2020 seems to be okay. (According to her mother. I am no longer the family tax accountant!)
