That particular box is asking for the date the partnership interest was acquired. It is needed when there was a "disposition" of the partnership during the year (e.g., sold your interest or partnership ended). This is typically "triggered" in TurboTax when you check a box for that on the Describe the Partnership screen that you see about 8 screens into the K-1 entry.
Also, be careful about your interpretation of that "Statement A Passthrough Entity Reporting". It may be using "passthrough" to mean passthrough to you the partner, not passthrough to the partnership from another entity. If that Section 199A "Statement A" doesn't list amounts for more than one entity (e.g., your "main" partnership and another "passthrough entity") you may not need to "split" the K-1.
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