This is a very
common occurrence when adding income to your return. Please see the TurboTax FAQ "Why did my
refund drop when I entered a second W-2?" below for a good explanation. Note -- it doesn't necessarily happen on the second W-2. Once you've reached the standard deduction threshold, it's common for the displayed refund to drop.
https://ttlc.intuit.com/replies/3798403
