I think I figured out what is going on here.
In EasyStep, TurboTax asks you "Did you work while you were a student during those 12 months?". I had clicked "Yes", because (in my case) I was technically working but paid through fellowships (reported on 1098-T) and not W-2 income. Since I received more than $500/month in fellowship payments, I selected "Yes" for the subsequent TurboTax prompt. Then, on the next page, "We Need Some More Info to Calculate Your Credit", I entered in 12*500 = 6000. Note that TurboTax, under Step 2, says "We'll then take ***'s income of zero and update the qualifying income used to calculate this credit".
If you complete EasyStep this way, then Line 4 on Form 2441 ignores the box to estimate deemed income from being a full-time student and instead pulls directly from the Earned Income worksheet (which is zero in my case).
I fixed the problem for my tax return by instead selecting "No" for the EasyStep question "Did you work while you were a student during those 12 months?". Now, form 2441 enters $6000 on line 4 and my tax credit is calculated correctly.
