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You are right, It appears the issue is tied to the number of months the vehicle is in service. For example:
With a purchase date of January 1, 2024, and all other factors unchanged, the business-use percentage calculates to 58% as I expected, which qualifies for the preferential depreciation method.
With a purchase date of December 30, 2024, under the same conditions, the business-use percentage drops to just 4.97%, which does not qualify for the preferential depreciation method.
This suggests that vehicles purchased late in the year are effectively penalized, since the business-use percentage must be much higher—closer to 95%—to be recalculated by TurboTax over 50% threshold on Line 9 of Turbotax Car and Truck Expense Worksheet (that gets put on 4562).
I also noticed that answering “Yes” to the question “Did you track the miles driven for personal use?” eliminates the purchase date as a factor. However, in TurboTax this option only appears when the vehicle is newly created. If the vehicle is simply edited, the question does not show up anymore—requiring the truck to be deleted and re-entered in order to access it. Screenshot here (edit&new workflow):
